In terms of his ability, both in mic work and in ring work he is in the top 10 in WWE history for me but he didn't quite get to the level he had the potential to.
If I look back at his accomplishments he did a lot, 2 time WWE Champion with the longest reign in 25 years and he also won the Intercontinental Title and Tag Team titles, as well as the ECW Title. He successfully defended the WWE Championship at a WrestleMania (but not in the main event) and took on the streak last year. But for me in terms of his accomplishments it doesn't quite put him as one of the best of all time for me. Most of his accomplishments I could say the same about Diesel for example, Diesel had a year long reign as champion and successfully defended it at WrestleMania only to lose it and then take on The Undertaker at WrestleMania.
CM Punk lacks a Royal Rumble win and a main event match at WrestleMania, and when I say main event I mean the last match on the show. None of this is a fault of his own, he could very easily have won both the Royal Rumble and actually main evented WrestleMania during his run so I don't put the blame on him at all.
Top 30 in WWE history I would say, with another 3 to 5 years he could have been top 20 and he still might do that. Personally I think he will have a year away and then return next year, I was one of the people who still thought he might return before WrestleMania but he needs a big break.
I remember Chris Jericho took a break from 2005 to 2007 and he was around the same age as CM Punk at that time and he came back completely fresh and rejuvenated. CM Punk needs that sort of break, probably to the point where even a year away wouldn't be enough for him.