What are your thoughts on the effectiveness of this ability?

Then, what are your thoughs about Frenzied Regen vs. Regrowth/Rejuvination?

I've used it to great effect when I'm low on health in bear form, the mob is low on health, but hitting me harder than I'm hitting it, and I dump my rage to buff up my health to outlast FTW. I also like to use it after killing a mob, to get that much more health before I transform back to caster.

It's not even close to regrowth or rejuv, but it's very nice. I've used it to give me that much more of a buffer on health so that when I transform to caster, I can get my regrowth/rejuv off with less worry.

I have found this only beneficial when tanking for a group. I understand it may be useful in pvp and such, but as my build is resto, I don't think I have the points to really realize it's full benefit any time but when I have many targets pounding away at me. The only time I have used it effectivly is on two occasions,

one while pushing some low guildies through VC and they aggroed honestly maybe 20 of the little boogers on me, our 15 priest valiantly tried to heal me, but with over 5000 hp she could do little. I used FR in that case to gain the edge for a SS/NS HT10 with a rejuv prior to dropping back to bear and swiping them out. LOL, never seen so many 18s streamin up in my life, thorns ftw :P

The other time was when I was tanking a stealth lbrs instance run and our healer went down. As I was building much more rage then I was spending holding the aggro, I again used FR to recover from about 1800 hp for a SS/NS/HT10 then a BS with a tranquility ftw.

It has it's place but I know I need more practice to use it most effectively. As for comparing it to our HoT spells, I don't honestly think they are in the same league as they don't replace each other in any way.

Also I was thinking about which classes in PvP it would be most effective against. Can it be debuffed by a priest or shaman? Is it more effective when fighting against a rogue or a mage or another class?

I think this has to be one of greatest abilities so far since Innervate. Frenzied regen saves a smart druid the mana required to heal, and shift back into Bear to attack. I don't know if it can be dispelled but when I fought a Shaman, surely enough, Regrowth, Rejuv, MotW was dispelled, but when I used Frenzied Regen either he didn't try to, or just couldn't.

It's not as effective as Regrowth + Rejuv, but like mentioned before it definately gives that buffer of health to mitigate damage being dealth and give us more time to regen the mana lost casting initially and shifting. Without this skill I probably couldn't handle some situations the way I can now. I was able to beat a 60 Warrior at level 54 without rooting + moonfire spam by literally out tanking him.