Oh yeah, the iphone 4 is renowned for its amazing, faultless signal 
All i'm saying is that your 1 building and 1 experience barely represents the scale of their coverage.
How do you know its only one building I work in, ok lets talk CS services on T mobile whats your experience of them ? Don't slag the iPhone off because mine has never dropped a signal in 12 months !!
I work all over London and every time I go into shop i.e. Sainsburys or Waitrose no signal . I got rid of them 3 yrs ago, my mrs daughter has nothing but problems and she has a BB due to lack of signal and sluggish internet on T mobile!!
Since the combination of Orange/T-Mobile, it can cause some problems if you haven't set your phone up properly. Basically, you can set it to 'roam' between both networks, to get the best of both for GSM signal. But at present, Orange only supports 2.5G (Edge) for T-Mobile roaming, ie. 128k (slow) internet. If you set your network selection to be manual and force it to be T-Mobile, you'll get the 3G benefits of T-Mobile (whilst comprising on using Orange's network).
I've had a very positive experience of both Orange and T-Mobile in the 5 years I've been with them, hence why I would happily recommend them. You've got to bear in mind - you don't call customer services to have a pleasant chat - its usually to moan/complain, so their quality of service will be reflected in your attitude towards them; be kind, polite, patient and charming works wonders when dealing with cmr services. Be an a-hole - and expect to be treated like one (not aimed at you, but a generalisation).
As I said, check the coverage map first

Vodafone offer "SureSignal" if you want guaranteed signal in your home/office.