There is some truth to both sides of the argument.
Based on his talent and ability, Rondo absolutely is as good as people claim. He's a top 2 PG in the league hands down, and probably a top 10 player overall.
However based on his every day production, Rondo is massively overrated. Frankly he gets outplayed far too often by opposing PG's who have no right to be able to outplay him. There are far too many games where Rondo plays with a "minimum effort" mentality, and he often plays with a selfish arrogance that seems to hurt his teamates more than it helps.
Rondo is IMHO the best player in the league at connecting difficult passes, seeing the floor and orchestrating an offense...and yet too many times I see him force up bad shots when there is a wide open Jeff Green or Jason Terry in the corner.
Rondo used to be IMHO the best on-ball defender in the league at the PG position due to his length, his big hands and his agility...and yet far too often he gambles for steals, allowing his defensive assignment to go right by him for an easy layup.
Rondo is one of the best PGs in the league at attacking the basket - with his strength and explosive quickness he can blow by anybody pretty much at will, and he's very good at finishig around the basket...and yet he barely ever attacks.
Rondo is one of the best players in the league at leading a fast break thanks to his end-to-end speed, court vision, passing ability and ability to finish...and yet far to often he slows it down and holds the ball for long periods of time, allowing the defense to get set.
The problem with Rondo is not his game, it's his attitude. Rondo is beyond confident - he is cocky. He knows full well that he can dominate any player in the league when he wants to, but he also knows that he can play with 70% effort and still put up his numbers...so he seems to believe that putting 100% effort is wasteful in a 'regular' game when 70% effort is sufficient to get the job done.
As others here have stated, when Rondo is in "Playoff" mode or "National Televised" mode his play is a massive contributer to our success. However, when he's in "Passive" mode he has little if any positive impact on the team - you could replace him with any decent PG in the league and it wouldn't make a difference. In fact, in those situations repalcing him with any decent PG may even make us better.
The problem here is that this season he has been in "Passive" mode far more often then he is in "Playoff" mode - I'd say the ratio is probably 4:1 if I had to hazard a guess. That means roughly 75% of the time he has a nagative or neutral impact on the team. This is made all the more obvious by the fact that he's had a negative on/off net rating for most of this season. This is why we have been able to beat good teams this season without Rondo.
Another factor to consider is that we have a number of guards (Lee, Terry, Barbosa, Bradley) who are all very skilled players, but we've never really had the chance to see what they are really capapble of because Rondo has played close to 40 minutes per game, so they have mostly all been sharing the SG minutes and a couple of scrap PG minutes. Barbosa is a 'spark plug' type player who's very skilled at exploding with short notice - but Lee, Terry and Bradley are all players who would probably play better if they got more time on the court.
It's not just the minutes either, it's the roles. Rondo was always the key ball handlder for the 35-40 minutes he spends on the court, and his way is usually to hold on the ball for a long time while a play develops, then pass it off to the shooter last minute. The other guards never got a whole lot of time with the ball in their hands, so every one of those guys might get a little bit more confident and comfortable with handling the ball now that they will be doing it more often. The result is that hopefuly we will have 4 ball handlers on our team rather than one.
so yes, Rondo is an outstanding basketball player. He probably is the most TALENTED player on our team, because Pierce and KG get a lot of their success from pure hustle and hard work - for Rondo a lot of the time it seems to come easy because he's just so talented. Because of all the above reasons though, I just don't think we will be tha tmuch worse off without him...at least until we get in the playoffs, at which time I've no doubt we'll miss him.