Tons of fun all the way throughout, with plenty of memorable action set pieces, engaging characters, and superb use of exotic locations.
The story in this fourth installment, and best of the series, is relatively straightforward and for some reason it makes more sense and is easier to follow than typical 'James-Bond-esque' films in this sub-genre. It's an absolutely gorgeous movie, with cinematography of exotic locales that uses interesting shots and framing; and populates sequences with eye-candy ranging from exotic sports cars to apocalyptic sand storms.
I also love the addition of Jeremy Renner and the fact they brought back Simon Pegg. My only minor quibbles are there's a bit of a lull between the second and third acts, and they could have used more Ving Rhames throughout. This movie also has the best delivery of the line "No shit" in any flick I can remember.
My favorite of the series, and although I've now seen it multiple times it seems I always can't wait to watch this flick again.