Having heard all sorts of horror stories about obfuscatory officials, long hours waiting in the cold, aggressive questioning and other such pleasantries, I thought I would write a few lines here on my experience at the US Embassy in London today.
I applied for a P1 (Artist/Athlete) Visa which was arranged through my employer via a US agency. While this may have made the process smoother, I have heard of other colleagues having a difficult time at one stage or another.

Having to call a £1.20/minute phone line to make your appointment is pretty galling, but since I imagine most people in this situation will have the costs reimbursed this isn't so bad. You pay your visa application fee (hooray for the strong pound!) and get your confirmation emailed to you. My advice is to take the earliest available (8am) slot, as the Embassy fills extremely quickly. Even at 7.30am there was a queue of 15 or so people outside, and it's not warm in London in January.

After security scans similar to that of an airport you're admitted to the Embassy.
Applicants are strongly discouraged from taking in mobile phones, ipods or remote car key fobs, though there is provision for this. I wouldn't (and didn't) take any with you as this only slows things up.

A short wait later and you are called to a window, present your documents (obviously having triple-checked at home that they're filled in correctly) and have your fingers scanned. Read the Embassy website for more details on this. It's nothing drastic.

Assuming this is successful, you get an exciting piece of paper, and after a slightly longer wait are called to a different interview window, where a genuinely friendly and affable gentleman issued my visa.

I was out by 9.30am and wondering what to do with the rest of the morning!
The overall impression is that if you have everything in order, treat everyone politely and professionally then it will be reciprocated, and receive a friendly and efficient service.