His last legs were still good enough to be 4th in DYAR and 10th in DVOA last season. Keller was 32nd in both, and he's actually getting marginally worse (31st/30th in 2009, 27th/29th in 2010). I don't have the PFF numbers but I'd bet they'll tell you the same story.
Drop percentage has been pretty bad, but an interesting one that they recently tracked was Yards per Route Run and Keller was somewhere in the 16-20 range, (below guys like Daniels or Lewis). The thing I found interesting about that particular metric is that it was an interesting take on the correlation between conventional production and opportunities, and the list was pretty legit. Graham and Gronk were so far ahead of everyone it wasn't even funny, Evan Moore deserved more opportunities (which has been long known), ditto for Finley and Jared Cook.
With Keller, the stat basically inferred that he's not producing nearly what he should be when taking into account the opportunities he's been afforded over the past 3 seasons. While the formula was very basic, (yards/routes), the fact that FO's numbers wholly support some of the interpretations help make it a telling stat, (things like Fred Davis is probably pretty good, Heath Miller is probably overrated...etc).
The thing about Keller is he still produces yards, and from the TE position that's always going to be overvalued in today's league. He's going to get overpaid next offseason and it's not going to be by us. I thought we should have explored trading him before the draft, because the comp pick we're going to get for losing him (probably a 4th) isn't going to be as high as what we could have gotten by a some semi-desperate GM.