I've bought quite a bit of my photography gear in the US. Apart from medium format gear (esp. Mamiya!) it often works out cheaper. Lenses, camera bodies, etc., are global and will work anywhere; as Jonathon says above, chargers are normally switchable between US and Eupoean voltages (though a plug adapter may be necessary).
If you buy in person in the USA, then in most states you will have to pay sales tax (which is additional to the displayed price for the goods). It is my understanding that you cannot reclaim this tax when you leave the USA. If you have the goods shipped to you from another state, however, I believe you won't have to pay the sales tax.
When bringing items back into the UK, VAT at 17.5% will be payable. Duties may also be charged, depending on the category of goods (last time I imported some lenses, for example, there was duty of 6.7%). Any duties are added to the price of the goods you bought, and then the VAT is calculated. It can mount up! However, it can still work out cheaper than in the UK.
One point to watch out for is warranty. I've always taken the view that reputable manufacturers are unlikely to have defects out of the box, and so far the gamble has paid off, so I've never needed to test the warranty situation.
Not all goods are worth importing yourself, but some can offer substantial savings. Gitzo tripods, for example, can be had a lot cheaper in the US than here in 'blighty.
No reason why one has to travel to the US to import goods. I've bought stuff from B&H in New York, and had it delivered here in the UK within a week. Made quite a saving on Canon L lenses, despite the duties and VAT levied. Note that if you import, VAT is charged on the package value, and the delivery cost is included in this -- pretty mean of HMRC!