I recently purchased the Alesis GuitarLink Wireless, so I thought I'd provide a report on my experiences so far.
I haven't used it in a performance yet, but I've used it at home plus in a rehearsal held in a performing venue. The first time I used it, I experienced a very noticeable hiss. I switched back to the cable. A few days later, I thought about the possibility that the hiss was related to the channel that I was using, so I forced a resync, and I had no hiss.
Later, I noticed that my pedal was producing a similar (but lower volume, I think) hiss under certain circumstances. So that initial hiss may not have even been from the wireless.
At home, I noticed I started getting noticeable dropouts when I walked from my office (where I rehearse) to the living room, about 20-30 feet away.
During the venue rehearsal last night, I couldn't travel very far because my IEM is wired, but I did notice one dropout.
So, that's my report!