Should I enable TRIM in Crucial M500 SSD over Mavericks?
I'm not sure you are going to find a definitive answer. I have a MBP 2012 with with a Samsung 840 500GB that I keep pretty full. Testing with Black Magic, drive performance had cratered (around 150/50MB/sec R/W). I installed Trim Enabler, filled the drive by creating an encrypted disk volume, deleted the volume, and waited a couple of minutes. Black Magic was back to normal. That was a couple of months ago. However, I just ran BM and the numbers are back to ugly (150/50 R/W). I'm running Filevault. So clearly TRIM and GC isn't enough to maintain performance over time in some configurations.

I also have 2009 MBP with an OWC 512GB w/o Trim enabled that is run with even less free space and the numbers today are as good as they were after initial install. So clearly GC is sufficient with some drives.

On the other hand, I didn't even notice that my notebook performance had regressed so unless you have an odd workload, maybe the answer doesn't make any practical difference in most cases.