Rob's advice to have a good idea of what you want and the cost is sound. Friends of ours get ripped off every time they go because they simply don't research properly first. When we were there last October they paid $400 for a digital camera. We checked the prices when we got home and they could have got a much newer model here for about £130.
I'm not saying there aren't necessarily deals to be done, but I think there are some sharp operators out there who will exploit those who don't know any better. Don't pay their first asking price (or even their second). And don't fall for the "I'll let you have it for $X as long as you take it now" scam - they just don't want you to shop around.
Be aware that not all the models available here are available in the US and vice versa - in other words, make sure you are comparing like with like.
You also need to consider that it could go wrong once you get home and the problems that would undoubtedly present. And most purchases of this nature will take you over your Duty Free allowance. If you declare it (as you should) you will pay taxes when you bring it back into the UK and there's no real way of knowing what they're likely to be. If you choose not to declare it and get stopped, Customs can confiscate it.