I've just picked up a Sennheiser Freeport wireless system from my local dealre for £125. I'm doing a big gig over Bank Holiday weekend and wanted to take advantage of a big stage :D

Sound quality is very good, fantastic for the money. There is a difference when compared to my cables in that the wireless has a bit more treble and bit less mids. It's quite a subtle difference and you can adjust your amp to suit it if you need too. I have the transmitter on the lowest possible setting and the receiver output around 9/10 (which gives a slight signal boost). Crystal clear sound in the same room as the amp, and no issues with reception as I walked around the house(!). The receiver is housed in a metal case and looks sturdy enough for regular use. The transmitter is plastic, but looks like it'll hold up OK.
For a laugh I went down to the bottom of my garden noodling away and it sounded superb! Felt and looked very bizarre though! :P
I'm going to try it out in rehearsal next week prior to the gig, but things are looking good so far. If anyone's thinking of going wireless the Sennheiser Freeport looks like a great reasonable priced option.