You can buy just the apogee sensor for $140 bucks and then connect it to a good multimeter for readings or you can buy the seneye reef module which contains a PAR meter for $189. The reviews I have read on this have compared the PAR readings to that of an apogee meter and the readings have always been very similar.
It can help you place corals in your tank if you know the approximate par of various areas. If you change lighting it can help you acclimate your tank to the new lights. You can judge bulb life, and the differences between different bulbs, as well. Whether it will be used enough to be worth the money is up to you. I plan on buying a sensor and hooking it up to a multimeter. For $140 I think its worth it, at $300 not so much.
I will be buying one by Christmas myself. Me and a couple of other guys in my area are thinking of going in on one.
Some uses I can think of off the top of my head.
1.) Use it to test when bulbs are wearing out.
2.) Strategically place corals in their proper location by measuring the PAR levels in your tank.
3.) Determine if you have enough PAR in general to support the types of corals you want.