Tethers fine. No problems with calls, only ever drops out in blackspots as you'd expect (Browns Lane being one of them).
I was nervous about 3 (I was one of the early movers to them years ago and the service was shite), but trialled my old iPhone with a 3 PAYG sim to see how it was for a couple of weeks before I took the plunge. Might be an option to see if you get the coverage you want at home / work etc?
I have a iphone 4 on 3, I tether a lot and it works perfectly (MacBook) pretty damn fast as well. I upload some heavy stuff (graphics etc) and never had a problem. The iphone has never been the best at calls! Also I'm not in Cov. But Im around the country a lot and the coverage seems more than up to it, I wasn't aware of any bad press but then I haven't been looking....
I use 3 for my iPad and laptop fine signal in Coventry, even got signal in Wales but not so good on the line between Cov and New street but also have poor signal on that route using my virgin mobile. Nonetheless cheap and reliable. Hope this helps