The Near and Mello Arc had the potential to be a great finale if they'd instead made sure Light didn't know there were two of them and taken him down with a pincer attack. It'd also stay consistent with Near stating they weren't a match for L individually as well. However, Ohba decided to fuck things up.
Part 2 was a huge letdown compared to the L Arc. As a whole, it wasn't as shit as most other 2nd parts of Shonen series (eg. Naruto, Bleach, etc.), since there were still a few good strategies Light pulled off before he went full retard, but still a similar drop in quality from some very good writing with Light's conflict with L. It's still one of the better Shonen series (I'd probably place it in my top 10 Shonen), but the drop in quality between parts, as well as the L Arc not having a solid enough ending to just say the Near/Mello Arc can be ignored, would have me rate the series no higher than a 6/10.