My last 2 phones have had Orange Signal Boost (UMA). As I live in a rural location with poor signal, I couldn't live without it. They generally worked well, though my last handset (LG Optimus 1) sometimes crashed mid call - perhaps due to lack of processing power.
I am currently looking to upgrade my phone, but I am dissapointed with the range that Orange offer - currently only Blackberries and 2 Samsungs. Nothing with a large screen e.g. Samsung GIII, HTC One X or XL.
So for me, limited choice of handsets is a limiting factor.
Strangely you were running a competition for the HTC One X with Signal Boost back in July, but according to Orange the current HTC One X does not have this feature - a backward step in my view.