I had a t-mob web n walk a while ago, the only actual restrictions i felt were voip, and nimbuzz being blocked. Everything else seemed to work fine, they can't detect tethering etc.

If anyone can afford the upfront cost, the best network to be on is o2, but youd have to buy the n900 either sim free or cheap on t-mob. Then get o2 simplicity 12 months@20 a month, for 800 mins, 1600 texts and unlimited data. It is now really unlimited (no 1gb etc) so as long as you dont abuse it e.g. peer to peer it should be ok. They dont filter nimbuzz either. And you can get £100 off by going through quidco.

On balance i went with the t-mob combi 30+web though to have no upfront costs and not pay for two contracts. so slightly less minutes and 1gb data.