Mutants & Masterminds, second edition is an excellent product with only a couple of flaws.
It's a solid d20-based system, that takes the core mechanic all the way to it's extreme end, and does so beautifully. The core mechanic of rolling a d20 and adding stuff, then comparing the total to a target number (DC) is used for almost every aspect of the game, from attacks to damage to skills. It's elegant and pretty consistent.
The powers system is extremely accommodating. With a bit of thought, I have been able to reproduce every power and power set possessed by every fictional (or non-fictional) character I've thought of. From the miracles of Jesus, to the powers of a Green Lantern, to the magic talismans of the Jackie Chan adventures, it's been quick and fairly painless. The extras and flaws mechanics, combined with power feats, does a great job of providing nearly limitless customization; with a few additional flaws and extras (usually worked out by myself and the guy running the game, for really odd ideas) I've been able to do exactly everything I've wanted to do.
Unlike most d20 games, you can actually have all of the feats and skills you want, right now. And at exactly the values you want. Wonderful.
The PL cap system is brilliant, allowing a costumed human detective and a flying, super-strong alien to operate at the same level, fighting the same villains, while still feeling like completely different characters. It effectively allows two different styles of progression, one where you get more rounded and another where you gain more raw power, and you can even combine the two.
The example archetypes included in the book are pretty good starting points, though they seem to make greater use of the Impervious extra than they really need to.
There are a few highly unbalanced combinations of powers. These include being Concealed from all senses (except touch), being completely incorporeal, being able to fry someone's brain from your bomb shelter on the other side of the planet, and several others that are pretty obvious when you sit down and read through them. These powers can still be fair and fun, with a little bit of common sense and utilization of the existing flaws.
LIKED: Simple elegance of the basic design. Extremely flexible powers system, coupled with fun and interesting drawbacks / complications mechanics. Brilliant Power Level system to keep most things roughly balanced.
DISLIKED: Unique subsystem for most powers; while this makes each power feel special, it makes it very difficult to remember how many powers work during the game. Movement powers quickly become dominant; if you don't have any movement powers then you are at a serious disadvantage once things get mobile.
[5 of 5 Stars!]