There will be several element to what data rate you get.
The first is how much capacity the sites have coming in, most as standard, before the present upgrades began, was one link, most T-mobile sites are now going to 4 links, I upgraded 2 sites today. There is no way of knowing what the capacity is of the site you are connected to. You will get HSDPA on a site fed by a single link, so this site has only a very small increase in capacity gained form using HSDPA, so you will see 3G ish rates.
The next is how much voice traffic the site takes, voice will alway take precidence, so if a site has high voice calls, this will have an impact on data rates. I upgraded a site on Vodafones network and the site was so busy voice wise, that it was impossible to connect via HSDPA to test the site.
The next thing is not all customers are equal, in Vodafone only coporate customers get access to full data rates. We had to be given the same access to enable us to test the network and the difference was very noticable. Not sure how T-mobile work this, but if the do the same and you have a coporate customer nearby, then that will drag your data rates down.
The networks are quoting 7.2M, but none of them have enough capacity going into there sites to get anywhere near that.
They will only get to that level or higher when the go to fibre straight into the base station, only Vodafone have comitted to this and they also intend to roll out second carrier to all their sites inside the M25, so doubling the size of the network.
The best option really for the networks is to use 3G as purely a data network and push all voice calls onto 2G.
I would agree about pushing voice calls onto 2G. There does not seem to be any benefit of having voice on 3G as it takes alot more battery life from the phone.
3G for data and 2G for calls I say!