I'm not one for commuting but I usually use my MTB's as much as possible just for the burn.
Folding bikes like Dahons seem to be a metro favorite.
Cannondale Bad Boys are great Hybrids but I prefer Double Suspension rigs. If you really like to ride and have some fun than you may want to consider it. When I can't find a buddy to go XC mountain biking with, I usually ride over the GWB and play my own version of "Frogger" in NYC traffic just 'cause I like to feel a rush when I ride.
As far as Road Bikes (racing bikes), they just aren't fun or practical IMO. The skinny tires with extremely high pressure (+/- 100 PSI) don't allow for an "oops" factor. So if you hit a tiny pothole or a MONSTER pothole at best you have a flat but often times you're out of a tire (if tubeless), a tube, and sometimes an expensive rim but very often time, you can get thrown from the bike.
With a DS (double suspension) MTB with approx 4" of front and rear travel, there usually is no "oops" factor. Whether you hit a monster pothole or you unexpectedly hit a 6" curb because of some driver not seeing you, the suspension absorbs the impact with ease and you might have to readjust your sunglasses but that's usually about it. Plus if you feel like getting air and see a wall or something to jump off of, it makes the ride that much more enjoyable.
If you end up using a bike for more than just a 3 mile commute than you may want to consider outfitting the rear cassette with a "granny gear" for assistance with climbing hills. If you like to feel the burn or aren't going to deal with hills then Single Speed bikes is truly where it's at but make sure you bring an extra water bottle =).
Single speeds are usually cheaper becasue they don't have gears, shifters, extra chainrings, extra gears on the rear cassette and cables and being that they don't have all those extra parts, they are usually cheaper to maintain because they don't have all that stuff to break with everyday wear.
If you like to ride hard then definitely don't go cheap because it's no fun when you're about to take a curb and your front axle comes off but on the other hand if you have to chain this thing up outside and out of site then it will most likely get stolen if it is on the higher end. I know a few people too that broke their crank or bottom brack because they pedaled too hard up a hill which is BS. (inside joke for MTB'ers....so now they're strictly Downhill bikes...lol) so cheap can sometimes suck. Most bikes in NYC that are covered in duct tape amaze me because they are usually higher end bikes that people are trying to disguise as hoopties.
Don't forget about a bike lock. I like the ones that are "shackleless". The body of the lock is basically like a "U" when open and there is only a small opening for 2 links of a chain to enter and as you spin the key, shackle "threads" thru the 2 links so now the shackle is protected by the body of the lock and the chain links (or cable loops).
Those big "masterlock U-lock" types can be seen on youtube with a tutorial as to how easy they are to break. I buy hardened machined steal chains and use one of my popped tubes to slide over it to protect the paint from being scratched by the chain. All bicycle chains are too easy to break or cut.
http://www.amazon.com/dp/B00004Y8CA/?tag=reefs04-20
I like using a M.U.L.E. Camelbak. Holds lots of water and almost anything else.
Helmets... pick one that best fits your style. Most are the same as far as meeting minimum safety standards. The better ones will have full 1 year replacement warranties. My dog chewed one up and my LBS got me a new one. I was surprised but it was his recommendation. I had already passed it off as a loss in my own head. The more holes = the more ventilation (may be better in the warmer months). Less holes (BMX style)= less ventilation (may be better in cooler months)
If you really get into it then some people will outfit their pedals with baskets to slip your shoes into or on the higher end will add clipless pedals which means that you have to buy special biking shoes to click into your pedals. Both methods make it easier to climb hills because you can pull your let up which in turn pulls up the pedal because they're attached as you push down on your other pedal and vice versa so now instead of only being able to push down to propel your bike now you can pull the other pedal up maximizing efficiency. Mountain bike shoes are much more practical than road bike shoes because they are easy to walk in and are very similiar to regular shoes some are like sneakers. Many road bike shoes require the skill of walking in the reverse stilleto heels kinda sorta.
And if u want to get stupid... Check out Bob Yak & Ibex trailers. I use my Ibex for doing grocery runs.
..
