On my intake to BA there were 3 who were from easyjet. All really top guys, very personable and positive people. I've not seen much of them since we joined (they do work hard on the 'bus) but from what I have heard from them they seem reasonably happy with their lot at BA.
Positives seem to be that pay is apparantly a bit better, though not hugely so I believe. The guys I joined with are all young, mostly single, and seem to quite like getting to nightstop in Europe. In time to come there's also obviously the option to do long haul, bid for your work etc which might be important to you.
The negatives I was told is that it's definitely harder work than what these guys were used to at easy. The Airbus guys at BA seem to be racking up over 800 hours a year which due to the chaos that is LHR means that most people bid off the fleet as soon as they can as it really is exhausting on short haul (so I'm told).
For my own part, I joined from a charter outfit onto the 777 and am pretty happy with my lot, though by my standards at least I work far harder at BA! 550 hours a year in my previous airline as opposed to 850 hours now and no the pay is barely different (long term I know it will improve for sure). I'm sure easy has a far more positive and fun culture than BA but that aside your decision would have to depend on your long term aspirations and how many more years you anticipate flying for.