correct, USB does not pass on TRIM, but all modern SSDs have their own internal GC that works independent of the OS, so it will be fine provided you have the drive powered but not doing anything for a while to let the internal GC do it's thing.
Any SSD will have issues if run heavily 24/7 because even TRIM requires a bit of down time to work. I do not know just how much idle time a drive needs, but suffice to say that my SSD has had no problems, and it should not be getting trim commands because the mobo is set in RAID mode (picked up a cheap 240GB Agility 3 on sale a while back).
To be honest I have not had a chance to bench a non SF SSD yet to see how it runs, but I have the cache for Premiere 5.5 set to my Agility 3 and have had no problems with it so far. Here is the difference I had:
Old setup:
i7 2600 3.1GHz
500GB system drive
1TB documents/scratch drive
1TB video/source drive
-Could only push CPU to ~60% load on renders (largely due to older 1TB drives)
-Long load times for both program and project
Current setup
i7 2600 Turbo OC sits at ~4GHz under full load with 8 threads
240GB SSD system/scratch drive
500GB documents/backup/export drive
1TB RAID1 for source materials
-CPU load now consistently up in the 90+%, while at ~3.8-4GHz
-Much faster program load time and export
-Turbo boost aparantly works just fine with multiple thread loads... provided that you keep the CPU frosty
-Still slow project load time due to older slow 1TB HDDs housing the source material, but once it is in Ram the projects fly, and I feel much better about the RAID1 redundancy for my project files, nothing would be worse than telling a client that their project is lost :/
Obviously the workflows and times for export vary wildly depending on exactly what the project calls for, but seeing that CPU usage jump from ~60% to a consistent 95% speaks volumes to the usefulness of a SSD. And that is a cheap Agility 3, I could only imagine the performance from a better quality device.
Other system specs:
Win7 home 64bit
16GB DDR3 1333 (considering moving to 1600, but not sure it would make much difference as I am already capping out the CPU on export and editing) (I have only had one project so far that used up all of the ram, sadly I cannot jump up to 32GB unless I get expensive 8GB sticks, and move up to Win7 pro)
GTX 570 to help speed up color correction and other transitions within premiere
Hyper 212 Evo CPU cooler with 2 silent Enermax fans in push/pull. Idle temp is 1C over room temp, full load on Intel Burn test hits 3.9GHz and 58-61C (depending on the core you are looking at)
Hope that helps