I've been mildly disappointed in JET, but I think it depends what your expectations were to begin with. For the most part he's been more of a defensive liability than I expected, but otherwise I've been okay with him. That said, I was expecting him to be the 7th or 8th best player on this team, and not an all-star or 6th man of the year. And that's what he's being paid.
That said, I'd prefer the Celtics instead look to spend it on players coming off rookie deals, especially second-rounders that have succeeded, or International players, and maybe look to split it amongst two players. In essence, that's what they're paying Courtney Lee, and he'll be a fine piece of this team for a few more seasons.
Assuming Pierce and KG don't retire this offseason, I'd love to see them target two such players with the MLE and one with the BAE. This team is more than one piece away, and it might be worth trying to catch lightning in a bottle more than once.