I've been using an Incognito on Boost for about a month now (switched from Verizon) and I'm happy with the service and the phone.
I installed Opera Mini on it and surf the web daily (when I'm bored at work). The web speed isn't bad. The longest I've waited for a page to load is probably about 10 seconds, other than that, they load up in a couple of seconds. My only complaint is that I can't watch video other than youtube. Some sites have video posted from other services and they don't show up on the page (unless I'm missing something).
I'm not really big on apps so I haven't tried installing anything else on the phone.
I've always hated text messaging because my old phone (a Moto Krzr k1m) didn't have a keyboard. The QWERTY keyboard was the first and foremost thing I was looking for on a phone. The keys take a little getting used to and it would have been nice to have the number function keys in a cluster for dialing instead of across the top (like a normal keyboard). It makes it awkward to type in phone numbers that way.
The camera takes good pictures and the only drawback is that it doesn't have a flash.
Here's a pic I took with the phone of my newly adopted dog, Scraps:
I resized it for the board.
I haven't tried out the mp3 player (don't have a micro sd card yet).
The service has been good for me so far. No dropped calls, missing or late text messages, etc. I tested the web coverage on a train ride a couple of weeks ago from Ronkonkoma to Penn Station without a hiccup in service. In fact, I had a good signal the entire ride.
I will recommend that you immediately buy a case and screen protectors for the phone--the outer display scratches easily. After an hour in my pocket (with nothing else), the screen had minor surface scuffs. You can buy a case and protectors for under $10 on eBay.
Other than that, it's a great little phone (though I do plan on buying an Android when Boost gets one for CDMA).