Sometimes I've been charged the duty (around 25% of the value, I never quite worked out how they calculate it), sometimes the parcel slips through the net and I don't have to pay. Yes, it closes the price gap somewhat, but it's still a good deal cheaper on most items. So it may not be the fantastic offer it first appears to be, but make sure to do the maths and work out how much it is likely to cost you and you can decide on that basis. A reasonable rule of thumb if you can't be bothered is to do a 1:1 translation from dollars to pounds, so if it costs $199 you should bargain on it costing you around 200 quid, including postage and customs duty. It may be less that this, it may be less by quite a chunk if you escape customs fees, but it gives you some idea for comparison.
Beware, however, of buying items illegal or unusable in the UK, including air bottles (DOT approved bottles cannot be filled or used in the UK, they must be HSE approved) and electronic markers/trigger frames (burst fire/full auto is illegal in the UK, and turbo is best avoided for the same reason.) Also think about support and parts - if necessary, order a full parts kit at the same time, it shouldn't add to the postage cost and could save you some headaches.