I decided to get a corded electric one earlier this year.
I had wanted to go down the compressor route, but the space they take up, and more importantly for my budget, i couldnt get an impact wrench and compressor.
The cheapest corded one you can get in the UK is a Clarke CEW1000 at about £70, however whilst trawling eBay i found a 'Timbertech' impact wrench from Germany with a european plug. I bought it for £44 incl delivery and it included a UK adapter too.
I've used it a number of times over the last few months, and it's paid for itself already. VERY handy.
Sadly unavailable on Amazon now, and none on eBay either.
http://www.amazon.co.uk/Timbertech%C2%AE-ESSR01-Impact-Screwdriver-Carry/dp/B0036DVXXG