My 3 & T-Mobile Experience
I have had my 3 dongle for a year and tried the T-Mobile offering. T-Mobile at the time of taking it offered the "if it doesn't work where you live bring it back" and that's exactly what I did and they did it without any fuss (Warrington). Install was fine, coverage wasn't good where I needed it.
For a client I also got all the main PAYG free SIM only and stuck them in a nokia to see the 3G coverage (Kent). This worked well and allowed them to settle on a network that worked everywhere they needed it before forking out on a contract.
So 3's service and probably the same for all:
1) Not good when in transit - e.g. on trains, cars etc. You spend all your time looking at the connection monitor to find out why outlook won't send, MUCH less productive than being offline. Fortunately Virgin West Coast have free wifi for 1st class and payfor in standard (it is t-mobile and google thinks you are in germany).
2) Connection time is painfully slow, i.e. from clicking connect to actually being able to use the connection. With a laptop on battery on the London to Ashford train you want to do a quick pick up whilst at good coverage stations. No chance, normally well over a minute to connect. Why can't it be seconds?
Specifically for 3
- when no 3G reverts to Orange coverage which is nice (2G so slow, but fine for emergencies on the side of a hill in wales when someone server as abended).
- When i took out the service I also took a 3 handset, but 3 wouldn't let me use my handset as a mobile BB connection. Therefore needed an extra bit of kit for the few times I need to connect - grr.
- Having a 3 handset is useful as it tells me whether it is worth (signal wise) booting up the laptop where I am or whether I need to find "higher ground".
Well worth it for me as an insurance for home working if BB goes down and occasional out in the field usage.