(Better back up now if you don't want to be here...)
More fun than plants and cheaper than fish, and not much more trouble.
They're very even-tempered and won't bite unless you mishandle them really badly; even then, most will just nip you to remind you that they have feelings too. (This assumes that the gerbil has not had a life of being hugged and squeezed and called George... beware a kindergartener's gerbils.)
They're cheap and easy to maintain.
They're fun to watch, and have the wildly irregular sleep cycle of a college student (sleep for a few hours, then get up and run around madly for a few hours). This could also be a disadvantage, except that gerbils very rarely vocalize (only when someone has just annoyed them nearly enough to get nipped) so if you take out anything they could possibly make noise with, it's not a problem.
Plus they make great document shredders.
Don't expect your gerbil to relate to you the same way a dog might, though. To a gerbil, you're just landscape. (Granted, you move around, but you're clearly far too big to be another gerbil.)
The best way to get a gerbil is for free. (There's no such thing as a free pet, of course; I mean not having to purchase the critter.) Find a school-age child. Ask her (or him) if anyone in her class has gerbils. If anyone does, they probably have two; parents are too willing to believe that "one gets lonely". This family probably has more than two, in fact, because gerbils of the same gender tend not to get along at first sight, causing most owners to believe that the gerbils are going to kill each other. (They are probably just trying to establish a dominance pattern, and will settle down eventually, but it is rather nerve-wracking to watch.) Gerbils of opposite gender reproduce at every opportunity. This is how to get a young gerbil for free; just make sure it's weaned and old enough to eat crunchy food.
The second best way to get a gerbil is to get one from a pet store. Don't get a fancy solid-colored gerbil. (They come in black, grey, cinnamon, white...) Much breeding was necessary to accomplish this coloration: almost necessarily inbreeding, which could have reinforced unhealthy traits. The "normal" or agouti colored gerbils (desert-colored on top, white underneath) are quite possibly also rather inbred. Don't buy one that seems malformed or has a kink in its tail and it should probably be all right. The only thing I have known gerbils to get sick of is the runs (from eating spoiled lettuce); just make sure the base of its tail is dry underneath. If the gerbil looks sick, don't buy it, of course.
The part that is going to cost you money is the cage. Get a 10 or 20 gallon glass aquarium and a wire mesh lid (something less flimsy than a window screen). Don't get one of those cutesy "gerbil habitats". They're made of thin plastic, and gerbils can scratch or chew their way out well within the average gerbil-lifespan. (Besides, they cost more.)
The other part that's going to cost you money is everything else. These things will be all used up or get chewed on enough to need replacement eventually; bear it in mind. Get a water bottle, and if it's made of plastic, get a metal guard to put around it. Make sure you'll be able to hang the bottle in the cage and still keep the lid on securely. Get a bag of wood shavings (aspen or pine, not cedar) to put in the bottom of the tank, and a food dish to keep the food from getting mixed in with the wood shavings. (If it's a plastic dish, the gerbil will chew through it sooner than you think, but it won't be as much of a disaster as the water bottle would have been.) Get some gerbil food. Try to get a kind without too many sunflower seeds, because gerbils really pig out on those and could get fat.
If you want, you can blow money on fancy toys, but gerbils only really need one thing (besides food, water, and a place to live). Their teeth grow constantly, so it is vital that you keep a gerbil supplied with something to gnaw on. A piece of hard wood with edges and corners will do well. When the edges become rounded, get the gerbil a new piece. You can also dispose of cardboard boxes, paper, and random other objects by putting them in the gerbil's cage, but these things won't really wear down teeth as well as wood.
There's not much to it. Make sure the gerbil never runs out of food or water. Once a week, put the gerbil somewhere secure (like a dry bathtub with the drain plugged, preferably under guard), dump out all of the wood shavings, wash the tank out if if it's starting to smell of gerbil (maybe once a month), dry it, put in an inch of wood shavings, and replace the gerbil. You should also play with the gerbil frequently so it doesn't get lonely and so it doesn't forget you exist. (If you are really worried about your gerbil getting lonely, buy two males or two females from the same litter. Make really, really sure they are both the same sex!)
Eventually, your gerbil will get loose, and probably sooner than you think. Many gerbils are good jumpers, and are quite capable of leaping out of an empty 20-gallon tank the moment your back's turned, so keep the lid on. When your gerbil gets loose, it will head under the nearest piece of furniture. (Iguanas go up, dogs go out, gerbils go down.) The absolute first thing to do is to close all exits to the room. If there is space for a gerbil to get out under the door, block it with textbooks or something. The next thing, if you have more than one gerbil, is to close the opening the gerbil got out of. Now you can settle down to actually catching the gerbil. They're faster than you think, and yours is probably terrified and is trying to remember how to get home. Some gerbils have enough of a gerbil-equivalent "sweet tooth" to be coaxed out of hiding with a trail of sunflower seeds. Some aren't that dumb. If you have a friend around, get the friend to scare the gerbil out from under the piece of furniture and towards you (for example, you wait at one end and have her poke a yardstick around at the other end). Be ready to grab it.
If you're thinking about breeding gerbils, have second thoughts first. You won't make money doing it. If you still want to, pick up one of the books your local pet store stocks and read it. Then have third thoughts. If you still want to, I suggest getting a pet snake, or an inexhaustible market (but there ain't no such animal); otherwise, you'll be up to your eyeballs in cute little furballs. Yeah. Gerbils are sort of Version 0.01 of tribbles. Be warned.
There are people at CMU researching rodent navigation. Maybe someday there will be robots that navigate as well as a gerbil. (Will they cost $7 at your local pet store, duplicate themselves ten times in a year, and survive a five foot drop onto carpet? Somewhat less likely. On the other hand, gerbils don't actually do anything useful, and a robot probably would.) If you know some other gerbil research links and want to see them here, let me know.
There seem to be a lot more gerbil pages on the web than last time I checked... no, closer inspection reveals there are just a lot of pages with the word "gerbil" in them, and a few that are actually related to gerbils. If you have questions my page doesn't answer (e.g. treatment of diseases), please check the Gerbil FAQ (Frequently Asked Questions). The (British) National Gerbil Society's page notes that the formation of an American society is being discussed, and also has a nice list of other gerbil pages. There are a few more gerbil links at AcmePet as well.