Thing is, with O2 they don't add bloat and I'm happy with their ROM, it's fast (ICS seems much faster than GB) and stable, which I guess we're lucky in that
I've set up a new sound set and tomorrow I'll be adding vids to watch on the train into London so will see how it goes.
Interesting though how it affects a Nexus device which I always thought was bare bones Android.
My Sensation is the standard 1.2GHz But with the XE Kernel, it performs at 1.5GHz... With a custom kernel, it's 1.8GHz (Although Not reccomended!)
Also, with undervolting I can save the tiniest amount of battery power and keep my device cooler at higher speeds or with overvolting I can acheive a better signal - none of which can I do with a Stock ROM.
I have used HTC Since the XDA BlueAngel and I don't beleive that HTC spend enough time developing and testing their software or hardware any more which is why there is such a call for custom ROMs and people on sites like this to develop.
I honestly don't know what O2 add as before I get a new phone, the first thing I look at is the ability to install custom ROMs onto it should I wish to
I know "3" add just about everything in sight which to some people is usefull, but to me it's junk which is why I ALWAYS remove their software for either unbranded or custom.
The biggest plus for custom ROMs is that patches & updates are released a lot more quickly due to many people working on it (The true beauty of an open source OS).
But if you don't have (or can live with) the problems that some others are having then I agree that there is no need to update or customise.
So, to finish my ramblings (And my overuse of brackets)... it comes down to personal choice, I like using or making ROMs which suit my own needs whereas others, like yourself, are obviously happy with the stock ones provided.