Rondo is not having his most productive season but he is still among the elite defensive PGs in the game, and in my opinion the best. Let's look at what he does well:

Quick hands - Rondo has the fastest hands in the league
http://www.youtube.com/watch?v=5uok4oLRtek&hd=1

Great anticipation - Rondo has a lighting fast reaction time
http://www.youtube.com/watch?v=NNmhNM0ESRU

Hyper-awareness/craftiness - Rondo is always alert
http://www.youtube.com/watch?v=BCAqS2qUHto&feature=BFa&list=PL363BEFDA74825746

Range - Between his long arms, spacing, and willingness to gamble...Rondo covers a lot of space
http://www.youtube.com/watch?v=OALgBxHNehQ

Versatility - Rondo can stick a man 1-on-1, roam and create havoc, and defend anywhere. The only defensive tool he lacks is strength and he's learned to compensate for that extremely well
http://www.youtube.com/watch?v=4f4ClgZgSOM

High Basketball IQ/Hustle - Everyone complains that Rondo gambles but the thing is... he's a great gambler. The odds are in his favor. Here's the Jason Williams steal:
http://www.youtube.com/watch?v=vvcbMNJQVbI&feature=results_video&playnext=1&list=PL363BEFDA74825746

Look at Rondo's position at the .07 second mark, he's in position to make a play on every passing lane. As the play progresses he looks like he's lounging, but he's still in great position. The instant the ball gets free he's already sprung into action (at the .10 second mark he's already in his sprint). Even though Williams is ahead of him, he takes a chance and dives through his legs to get the ball. That whole sequence is amazing.

Even when Rondo isn't making those types of sensational plays, he's a guy that opposing offenses have to be aware of because he is the only PG that effectively controls space (and not just his man). That's why he's able to get in position to rebound and help out so much. Comparing him and other PGs defensively is like comparing a safety with corner backs.

I hate that he lets his man go by him so often and that he often loses his man on cuts. But given how much effort he puts in night in and night out I don't hold it against him for conserving his energy for when it matters.

Last game Avery didn't play to his normal standard defensively. That's because his role was more complicated and he was trying to run the team (something Rondo does every night). Jameer Nelson blew right by him more than once and no one said a word. Yet whenever it happens to Rondo it's always because he wasn't trying or he was trying to get fancy. That's not fair. And I have yet to see Rondo get beat by his man when it matters most (except when he's not playing visibly hurt like in NY when he didn't chase after Novak and no one switched on him).

At the end of the day, even if he's not All-Defense this year, he's been one of the most exceptional defensive players in his era. He's the second to last guy (maybe 3rd) on our team whose defense we should be complaining about.