Pretty much everything that has been said above.
However, oem android skins do have advantages to, there stock apps are far better and they do have some nice features over stock android e.g. Touchwiz smart features, although you can get most of these on the play store now.
Regarding bloat, tbh it isn't as bad as what it was like a year ago and ever since ics you can disable them without having to root apart from the main stuff I.e. Sense launcher etc.
And any bloat doesn't really affect the speed or smoothness these days.
Pretty much every custom skin has a far better camera app especially senses camera app. I still haven't found stock apps on other company skins, even stock android that is as good as senses stock gallery, calendar, browser etc. And some 3rd party apps aren't as good either, only the browsers are a lot better when it comes to features, although the note II touchwiz version of gallery looks superb.
The only thing I wish I could change with sense is the theme throughout, much rather have the stock android look. Mainly that is why I use some 3rd party apps. Many say the same about touchwiz to.
Updates is a huge advantage though. Most phones will only get 1-2 updates and it takes months for them to arrive.