The Focusrite Scarlett 18i6 is a solid winner in the "bang for the buck" category. I added this piece into my setup for laptop/mobile use, my primary music station has a Steinberg MR816x and a Mytek 96AD, both great pieces of gear, but significantly more expensive.

In a slew of sub $500 USB interfaces I chose the Scarlett based on features and on Focusrite's name; this has had bonuses and drawbacks that I'll get to in a moment.

My first consideration was size, it's a half rack space and not terribly deep, and even though it's not bus powered (you need a wall wart power supply) it doesn't require breakout cables either. The controls on the face plate are clear, easy to grab and move smoothly. This may not be the first thing you look for in an interface, but when design isn't done well it's definitely something you miss.
I mostly landed on this interface for the digital connections. This is currently the only thing in its price range that gives coax sdpif and ADAT/SMUX optical. So if you wanted to use more than 8 inputs or wanted to use a Lynx or Apogee converter in addition you have that option. Perfect, I like to stay flexible and not have to replace new gear constantly.

The important (some would rightly say most important) bit is "How does it sound?" Simply put, great! It's clean, low noise and detailed. I compared the DA between the MR816X and the 18i6 and while there were subtle differences I wouldn't say one was better than the other (I have in the past compared the MR816X DA with Mytek and Apogee and found little difference either). So this little box is playing in league with the big boys.
The preamps aren't anything to write home about, but they're clean and have headroom and they're always there, so use them with confidence! This is one area that the Focusrite name was good for. The bundled plugins were a nice little bonus, nothing to really add to my library, but if you're just starting out and don't have a lot of plugin's yet, the Scarlett suite will be a nice place to start.

The cons: No power switch. It's a minor kvetch, but having to unplug the power supply from the back of the unit or kill the power strip is kind of a pain.
I didn't run into anything more than a few minor issues that required uninstall and reinstall (who makes computer based music and hasn't experienced this?), but the number of people with driver issues from a company that should have this sort of thing sorted before hand is astounding.
If you use an external monitor controller you would probably want to turn the controller knob all the way up to maximize your gain and headroom right? Not here, it distorts when turned all the way up. It's definitely a nice feature to control your monitor volume from the interface, but if you have a big knob or something similar you're going to lose a little volume.

Bought mine several months ago and it has never worked well. Sound keeps on cutting out all the time. I have to close cubase, unplug the unit, replug it, restart Cubase, go back to where I was. Really annoying when this starts happening every 2 minutes. There does not seem to be any pattern to it. It can work well for a day, the go berserk every few minutes, then work well again.

The computer runs XP SP3, it's a new install on a new hard drive. Computer is dedicated to recording and not internet enabled. The only other thing on the USB is the Cubase license key.

Never had any problems with my old Yamaha Audiogram 6, it always worked perfectly well.

I contacted the folks at Focusrite, sent them files and readings. Their conclusion? PC's come with bad USB cards and I should replace (a perfectly good) the USB card.

I'm about to throw the unit against a cement wall, that's how crappy the thing is.