I hadn't seen the last two M:I films, so caught up with them in last few weeks, Rouge Nation first then last night watched tail end of GP, which had watched up too the climax sequences (last 40 mins or so).
5 infinitely better than 4, GP was to a certain degree a bit 'ridiculous' in it's 'end of the earth, last minute Bond from the 60's' kind of feel'. 6/10 for Ghost Protocol.
Liked Rogue Nation far more and the action/story was better handled and delivered. 8/10.
Am now up to speed for Part 6 this summer.
My current ranking of them would be
1) Mission Impossible
2) Mission Impossible III
3) M:I Rouge Nation
4) M:I Ghost Protocol
5) Mission Impossible II