I've watched a lot of cop series (e.g. Law & Order, Bones, Castle, A Touch of Frost, The Mentalist, Life, Monk etc). Numb3rs places itself firmly in the middle of the pack. It is a solid show, quite enjoyable and recommended if you like this kind of show. Don't expect something original though.
So, why is it not great? Well to start with it, just like most other series of this type, borrows heavily from other shows like it. Every storyline has been done before, but math is used to come to the same conclusions as they would by looking at evidence in CSI or by noticing stuff in the mentalist/psych, or talking to people in Law & Order. This is often done in a... somewhat convincing way to a layman like me, but I often got the impression that if I were a mathematician I would be screaming at the screen a lot. This doesn't really break the narrative, though, not for me anyway. There are some episodes, however, where the writers betray an unbelievable ignorance and almost disdain for the group of people they are trying to portray (e.g. the hacker episode, the online gaming episodes, and of course any episode with a serial killer). Hardly unique for this series, but that's hardly an excuse. Overall the stupidity of the episodes is held at an acceptable level though.
Something which can be seen as both the main strength of the show as well as its main weakness, is the lack of extremes. Like Law & Order for example, Numb3rs avoids a lot of unnecessary drama that other cop shows (or just shows in general really) are prone too with characters fighting over irrelevant things that put them or their relationship in danger. On the other hand neither does the characters evoke truly strong feelings either. At least not for me. I did not hate or love any of the characters in Numb3rs, I liked them, I was not annoyed by them. They were solid (Megan and Larry my favorites, I also liked how Amita was portrayed most of the time). It would have been easy for them to make Charlie far more eccentric or his relationship with his brother far more loaded. I'm uncertain what to think of that actually since I for the most part found it nice to avoid the drama, but I still feel like not doing it did make it a slightly more mellow show.
Most of the episodic shows like this one usually has an overall storyline (perhaps an arch-villain they battle from time to time), but Numb3rs doesn't seem to have that. This could easily have left it feeling like the show had no direction, but I think they managed to avoid that by letting the character development putter the overall story along.
I'm repeated myself, but overall it was a solid, if unoriginal, piece of storytelling. I enjoyed it.