Depends on what you're trying to get out of them.
If you are still fairly new and you only have a few weapons at low levels, you're generally better off buying lots of Veteran Packs instead. You'll get a good selection of stuff, and quickly, along with a good stash of consumable items to see you through.
Once you have all of the uncommon items at high level, then PSPs are worth getting, as you're maximizing the chance that you're only going to get the valuable, Ultra-rare items from them, instead of spending 99,000 credits in order to get what should be a very cheap rare item.
Edit: Ninja'd. Almost thought I'd got it that time.