My question comes in two parts. Firstly, I'll ask:

Are USB mics any good? Not necessarily certain brands that have flaws, but overall flaws that come with the fact that they use USB. I've heard about massive latency issues, but the mic I'm looking at comes with a built in monitoring solution (the Shure PG27USB comes with speaker out that supposedly shows up on the computer as a separate audio output). I don't intend on using lots (if any) of effects, all I want is nice quality.

Secondly, I'm running Linux. That there rules out 50% of anything USB. Is anyone using this mic with a Linux box? I'd like to know, before I spend $250.

This is my first mic, for which I intend to mic an amp with, so any help would be appreciated.

For $250 you could get yourself a nice Interface (M-Audio Fast Track II usb) that connects through usb and a Shure SM57 and get far better results. Can't really speak for the second part, although I'm sure googling if the interface works with Linux will yield the needed answers.
ESP Horizon NT-II
Schecter Hellraiser C-1
Peavey 6505
Orange PPC412
Maxon OD808
ISP Decimator G-String
Boss TU3
well technically it's regular mic w/ D/A converter (aka sound card), so it could be pretty good, depending on what mic and what D/A converter they use. On the other hand, I think it would be better to buy proper sound card for your computer (even some intergrated sound cards are fairly good, on desktops especially) and get a regular mic. Latency wise....you said you're using linux, so i wouldn't worry about it that much. If you're familiar with JACK server, you shouldn't have latency issues. Windows wise, check if it has ASIO support.

Linux-support-wise.....well, that indeed is a problem >< It most likely would work w/ some generic sound driver, but the obvious problem w/ that is that it's generic driver and it can't really use any fancy functions the mic could have (e.g. 96KHz sampling rate)

I would personally either find a way to test that mic first (like some shops here have return policy where you can return the stuff you bought within 30 days for full refund) or get regular mic (and possibly new sound card as well, depending on what you currently have).
linux??? go windows or mac please... usb mics lack preamps go the audio interface and condencer/dynamic mic way...

M-AUDIO Fast Track=100$ish
AT2020 non usb=100$+ish or sm57 = 100$
USB mics are generally pretty good, but that Shure is absolutely not worth the money....there are much better mics available for much less.

For that money, you could get a basic but sold interface and a great mic like the Studio Projects B1.
Thanks heaps everyone.

I'll go with the suggestions of @Mopy and @binting, and get an audio interface and mic. The Shure does come with an inbuilt preamp, but for the sake of expand-ability, interface and mic should be best.

Have a good day everyone.