If you don't mind me asking - what did you end up paying compared to the sticker price? The problem with most services like Carwoo is that you can get a much better deal yourself with a little bit of knowledge and some time.
This is pretty much the car that I bought: http://www.marinadelreytoyota.com/vehicle/specs/toyota/corol...
One of the first dealers I visited in person told me the sticker price of $18,050, and I didn't think much of it since I thought they would easily lower it. At the desk, they quoted the sticker price and wouldn't budge until we walked out and were already halfway down the driveway, then whispered a price of $16,800.
Second in-person dealer, my dad called a friend and asked for a dealer recommendation. He went up to him and talked about all their mutual contacts and asked for his best price and was quoted $16,400 and that he couldn't go lower than that.
I sent a counter offer to one of my Carwoo dealers and got my car for $16,200. I also got a $1,000 college graduate rebate (which I would've gotten at other dealers too).
To skip all that running around and haggling, I would feel more at ease sitting at my computer and clicking "Accept deal" and paying the $49 for the service.
Edit: Before using Carwoo, I tried to get a quote from a local dealer through their website. I guess I skipped some fine print because they ended up leaving me voicemails and emails for the next 2 months.
At the end of the day, if you try to do it yourself, you'll spend a lot more time (especially if you drive to a few dealerships).
Plus, if you do it yourself, you'd have to give your email address and/or phone number to a bunch of car dealers. And, don't take my word for it, take stjarnljuset's in the post above this, if you give a dealer your contact info, you'll be getting spammed for quite a while.