Just came back from MI:5. Thoroughly enjoyed the experience and had the most fun out of any summer blockbusters I've seen so far.
As SamuraiBlue stated, there are many plotholes/inconsistencies and what not. But it really didn't matter in the grand scheme of things. Yes, there are cheesy dialogues. Yes, there are convenient plot devices and zany gadgets. But this is what made the Mission Impossible franchise so great in the first place. Instead of trying to mask the zaniness with "realism" and "grittiness", MI5 embraces its inherent silliness.
The special effects are what really set the film apart from its competition (in the sense that minimal CG was involved). I genuinely felt that I was sitting through a mid-90s action flick during the film and since they were filmed "for real", you are on the edge of your seat through out. You physically feel vertigo in the famous aircraft scene and I actually was holding my breath as well when Tom Cruise was trying to retrieve a macguffin underwater.
In conclusion, I felt that the makers of MI:5 understood what the audience desperately craved for in an age of high-budget, CGI laden, and interconnected mega blockbusters. It felt like a breath of fresh air and a throwback to the 80s and 90s when action stars, not CGI or franchises, were the driving force behind mega blockbusters. Definitely check it out if you haven't already.
p.s. Proof that Tom Cruise did it for real!