First off, I love having 4 wheel drive. I'm a diehard though and won't own a 2wd vehicle (Restored Comet GT being the notable exception). When I see baja bugs around here my first thought is, "how could I swap a 4x system in that?"
So... I might be a little biased. That said I'm not sure they cost more to maintain- well not significantly more. I think more 4x people maintain better.
You've got a front diff, which needs to be maintained with the rear diff. You've got a transfer case which needs to be maintained less than the transmission or diffs (in my opinion). And you've got slightly more expensive parts in some cases.
Gas mileage you lose in 2 major areas- the drag created by having a larger air pocket under the vehicle, and the drag of moving the front axle parts around whereever you go, larger tires etc.
I get about 11.68 MPG when lead footing my SMB 4x EB around (6.0 PSD), which jumps significantly when I drive more slowly. How you drive has a bigger impact than the 4x4 (probably) and the 4x4 makes how you drive a much bigger impact (e.g. driving 80 instead of 65 will suck more gas with a 4x than driving 80 over 65 in a 2wd that's close to the ground).
As far as use- I really really use my Jeeps, but that's because I take them wheeling. I've buried my F250 with trailer in a field and walked out in 4x lo range. It's great in snow or on a gravel road for more control, but you have to realize you're not invincible (4x doesn't help you stop) which is where a lot of SUV owners get in trouble.
Being in a 4wd and seeing what it can do is probably the best option, second to that is probably look at stock 4x vehicles driving (YouTube). I was always amazed at what my Jeep Cherokee could do stock. Considering 4x is 12K to 15K or more you might invest some time or money finding a older 4x you can drive... hook up with a local 4x club maybe or even rent an SUV and find somewhere to test it.