I used UserTesting.com early last year after coming across a coupon for a free testing session. The concept is really simple. You enter a series of tasks, basically a script outlining what the user is to do on your site, and they match you up with one of their pre-selected testers. The tester connects to your site, and follows each task one step at a time. As they're doing so, their screen is being recorded as well as audio, so they can talk you through their thought process and provide opinions on what they're seeing.
When they finish, the video is uploaded to the UserTesting servers where you can watch online or download to your own computer.
The overall experience was amazing! It really opened my eyes for the first time and allowed me to see exactly how a typical user saw my site. Things I always assumed were easy and obvious were not. As I had more and more users test the site, various trends were revealed and it became very apparent what problem areas I had and what I needed to focus on.
The site is well designed (as you'd expect from a site that delivers usability testing!) and had many helpful tools. You can add notations within the videos, define exactly what type of user you're targeting (location, skill level, any special requests, etc), and more. They even offer services to create the test scripts for you, if that's not your thing.
If you're creating a consumer product, usability is a must. Being able to see real users using your site and providing their feedback is critical, and I'd recommend UserTesting.com to do exactly that. The testers are hand-selected, so you don't have to worry about getting someone who isn't able to follow instructions or articulate their feedback. I've done about 10-15 tests on this site so far, and they've all been fantastic. UserTesting also allows you to get your money back if you don't find a tester helpful, and also provide ratings and feedback on the testers themselves.
Interested in seeing a typical response video to see what you'd be getting into? I have one up on my blog: http://blog.kitchenpc.com/2011/02/10/usability-testing-part-2/