Pros:
It's nice to specialize your character.
Some of the perks add new cool things that weren't possible in Oblivion.
Cons:
Having a tree requirement to reach the top perks. I don't want to waste perk points on "novice destruction spells cost 1/2 magicka" because I'm never going to cast those spells again once I learn the better ones, and if I do cast them, they aren't expensive in the first place so who cares? Another example is the lockpicking tree where top tier perks make previous choices useless and obsolete. They should be in a skill column but not a tree.
You don't start to feel special because just to survive you need a constant investment of low level perks like the base level perks in armor, one handed, shield, archery, alchemy, enchanting, etc. Those first perks that have 5 perk upgrades keep eating up my perk points so I never seem to get anywhere "special". Those base damage perks should not even be perks, but be auto upgrades as you level that skill. A little bonus for leveling the skill itself would be great, but no we have to waste a perk to get a 20% increase, where we should get a 1% increase each time we level that skill in the base damage/value.
You have to train skills you don't use to keep advancing to earn more perks. Why? Why not allow a skill to go beyond 100? Even if its just 1% damage adjustment and horrible grinding, I'd rather grind one handed to 120% than train two handed to keep leveling up to get more perks. This would allow one to get all the perks of a chosen skill without breaking character. It would make sense that ok yeah I'm dumb, but eventually I learned every single perk in one handed by grinding one handed to 150% because early on I wasted my perk points. Or just make perk points awarded per skill so you can only use them in the skill they were learned in.
Why not a perk to advance in health/magicka or stamina? Remember in fallout if you didn't see a perk you wanted you could just level up here and now, or take another point for special? For skyrim maybe I just want more magicka or health, they should allow a perk that grants you that choice.
Overall I think Perks are better than in Oblivion, but the design makes it kind of difficult to fully enjoy. I start characters with intent of making them a certain class and fully customized, but always end up wasting all my perk points on basic stat modifier perks at the base of the main tree of light armor, one handed, archery, alchemy etc and by level 30 I am just a stronger version of the level 1 character I had with ZERO new skills learned from perks practically. I don't feel special. Maybe this is my own fault but I feel like the game kind of forces you to do this. It's like you can't reach the good perks quick enough because they require a high skill, and then the enemies level up and require me to keep tossing points in armor/one handed/shield/alchemy etc just to stay competent. By the time I'm level 60 I'm still a jack of all trades and can't get enough perks to master even a couple of trees without resorting to training or using skills I'm not interested in.

