AFAIK, they're legal in the APA. However, you're not allowed to use a jump cue in the APA (except in Masters play and if local by-laws allow), which means if you really want to use a phenolic tip, it would have to be on your playing cue. I don't think playing with a phenolic tip full time is a good idea (but I do know a guy who does, and he hits 'em pretty sporty).
Phenolic tips make a HUGE difference on a jump cue. I replaced a Triangle with a phenolic tip, and I went from requiring at least a foot for a full ball jump to less than 6 inches. My jump cue is a jump/break, and I swear it feels like I'm adding 3 or 4 mph to the break. If you have a jumper or a J/B with a regular leather tip, give a phenolic tip a try. I was amazed at the difference.