I think I'm pretty neutral on this, especially considering the fact that I work for a rival dev, but I enjoy the rune/mastery/summoner spells immensely. It adds a personal level of customization that the genre sorely lacks.
My only gripe with it is that the advantages provided by runes and masteries are hidden, and it's literally impossible to know what benefits an opponent has. This isn't as bad for things like "+damage" or "+mana", because those are things you can figure out by clicking on someone. The bigger problem is armor/MA penetration. I've played against a few opponents that I felt did way more damage to me than they should, only to check them afterwards and see they have a buttload of penetration. Another gripe I have is the XP bonus masteries; I've died, on multiple occasions, to an Annie mid that reached level 6 before me, despite me clearly having an advantage on creeps killed.
It's a flawed system in implementation, but I enjoy the idea of it.
ALSO:
Summoner's Rift (Autumn) is an atrocity on the eyes. The red line spell indicators blend into the terrain. UGH.