Rondo is one of the best point guards in the game. I think he's at the top of the second tier, the first tier being guys like Paul, Rose, Westbrook, and then there are couple of guys who are making a case to be in that group (Kyrie and Curry). Rondo's just behind with guys like Tony Parker and Jrue Holiday.
Overall, I consider Rondo one of the top players in the game under 30.
But he's hardly untouchable in my book as far as trades are concerned. He has a very unique game with very noticeable strengths and very noticeable weaknesses. That means his value to a team is very dependent on the team's makeup and the goals that team has.
For a team with top level talent competing for a championship, Rondo is invaluable. For a rebuilding team that needs a leader who can take on the responsibility of winning games pretty much by himself on a fairly consistent basis? Well, Rondo is going to disappoint you there.