Besides usability tests being a waste of money IMO you should really be careful even if you believe in them.
I would therefore get clarity on the following questions.
One of the companies I advise uses UserTesting.com, and it's awesome. Their product is a consumer-oriented web site with a very broad market (almost any adult in the US is a potential customer). UserTesting.com lets them make a revision to their site and test whether the change is helpful or not. We do a ton of conversion funnel testing to understand if the change has a positive or negative impact, but UserTesting.com helps us understand WHY the change is having an impact.
We've learned some interesting things that we didn't expect. For example, the site used to have a small Google AdSense block on some of the interior pages, and the users found it to be very irritating. The Google ads decreased the credibility of the site in the users' minds. Since we weren't getting much revenue from the ads anyway, we simply removed them. We would never have known this without UserTesting.com.
Two thumbs up.
I found usertesting.com to be useful and cost effective. I summarised my experiences here:
We use Usertesting a lot :) It really helps to see if you User Interface is intuitive enough, how clear is you message for visitors and what impression your site makes on them.
Plus it helps sometimes to catch bugs you'd never discover yourself. Awesome service - it's like getting in the head of your visitors.
Other tool you may want to use to monitor your site is getclicky.com
They allow you to see each visitor to the site in real time, check where they came from and what pages they are viewing. This all is on top of standard site statistics.
Finally, there is an amazing tool called CrazyEgg.com that shows you the heat map of your pages - the most clicked spots that make it apparent if you are succeeding in controlling users behaviors/funneling process with site's UI. See what they notice the most and what they don't see at all.
Crap service. Tried to sign up with them both as a customer and a tester. Their own web service kept crashing, phone number went nowehere, and the online support essentially told me that they don't care. Oh well...