As probably the only (long-time) actual Baltimore Ravens fan on this board, I can say that the answer to the OP's question is: Yes and no.
Joe Flacco is not the "best QB in the league" (that would be Drew Brees, hands down). And no, he's never been voted to a Pro Bowl, though to be fair, he has also largely played in a run-first offense (you know, Ray Rice, Willis McGahee, Ricky Williams, those guys). But while he's been very up and down (something that AS a Ravens fan over the years has brought me a LOT of frustration, trust me), he also delivered when he needed to this post-season. The fact that he outplayed (yes, outplayed) the likes of Peyton Manning and Tom Brady on his way to a Super Bowl, coupled with the fact that he matched Joe Montana's record of 11 TDs 0 INTs in a post-season. That's pretty damn impressive. And it isn't as if he was playing against shitty defenses, Denver and SF especially were two of the top ranked. And he still did a pretty good job of tearing them up. That Super Bowl was a complete blowout in all three phases of the game until the convenient "lights out" nonsense occured.
So again back to the question, NO, he's "not worth" 120 million, because quite frankly who is? What sports player honestly NEEDS to make that much money? But on the other hand, considering that's what the market IS for top QBs, and Flacco IS the only QB in NFL history to make the playoffs and win at least one game each postseason his first 5 years, AND considering he had such a great postseason THIS year, and the fact that he has just as many Super Bowl rings as Drew Brees and Aaron Rogers, (and Peyton Manning), that yes, in that light, he is "worth the money".
When the Ravens drafted him, my first thought was "WHO"? And I have not been his biggest fan at times over the years, as I said he's been erratic, great at times terrible at others. But now we have to find out just how much of that had to do with our former OC Cam Cameron, and how much of that had to do with Joe himself. Cameron was shit, that much is certain. His playcalling was atrocious, and quite frankly when the offense succeeded it was in spite of him, not because of him. So now Caldwell is calling the plays, and honestly he obviously called some pretty good games, starting with the drubbing we laid on the NY Giants late in the season. That SB run and win was improbable considering all the injuries the team had, and the three game losing streak late in the year, but they got healthy enough and pulled together, Flacco included, at the right time. It's worth pointing out that if we had gone with the O-lineup we played in the playoffs (and with the exception of the RG all of 2011 as well), we would have done better all along in 2012.
My biggest issue is does this deal work for the Ravens as far as cap space goes? That's what's most important. Even though I think he doesn't need to play anymore now that he's got his ring, if Ed Reed still wants to play, than he needs to be a Raven, period. Reed should play his entire career as a Raven just the same as Ray Lewis did. It would also be really nice to re-sign Paul Kruger, as he came on very strong this year, and it would also be nice to restructure Anquan Boldin's deal so we can afford to keep him around. Things like that, to keep the team as a whole competitive. The Ravens in general have a lot of talent, even with Lewis now retired. We need to make sure our young center steps in adequately for Birk, and we need to address the Left Tackle position (and by address it I mean someone OTHER than Michael Oher, who is a RT, NOT a LT). But really, the Ravens have the nucleus to stay competitive and get back to another SB in the next few years.
So in closing.....yes and no. No player is worth $120 million in my mind, and Flacco is no Brees. BUT, given that, Flacco is still good, and deserved to get paid, and if that's what the market says top QBs are worth, then there you go. He just needs to be more consistent in the regular season now, and hope that with a better offensive system, he can continue to play at a high level.