My druid is 88 (and heading to 90 very quickly) so he's only just unlocked this. I agree with people saying it's difficult to 'learn how to use it'. Of course, only being in 5-mans right now, I can't exploit the full potential of it, but I will say that it's saved my groups a few times. The issue I have is not knowing how many people I can get rejuv. cast on before I need to switch to Genesis (this applies if I'm just standing in Org or Shrine and healing random people -- it's interesting to see that it can hit almost anyone in the main areas of those cities regardless of grouping). Sure, it only takes 2.5 seconds, but genesis there, and you've already just about lost the first tick meaning the rest finish up in 2.1 seconds.
I guess it just feels 'too quick'. I'd honestly have preferred for it to spread rejuv. to other targets (again, I'm 88, I don't know the entire spec yet so I'm not sure if we have a tool to do that elsewhere) and then hasten it - perhaps put a cooldown on it to make it a raid cooldown (as opposed to giving it an exorbitant mana cost with no cooldown) and I'd feel like I could make real use of it. You know, sorta like old mushrooms, but remaining as a HoT (albeit a quick HoT).
Perhaps it's just me still learning the spec, but I don't feel that comfortable with genesis right now. I'm also really missing the old style of mushroom - even if it took time to setup, it allowed flexibility and massive burst healing either in a compact area or over several groups that are split (e.g. Lei Shen phase transition - I assume this would be a valid use for the old version assuming you were in range to trigger them all), compared to the new one that's planted near a target (I know it can be glyphed, but it's still only 1 mushroom and the healing on it seems highly reduced in it's new iteration). I do like that I can move mushrooms and keep the healing though, definitely a bonus but overall prefer the old style, and genesis really doesn't seem to fit the same role, there's an emptiness of sorts where mushrooms used to be. I'm sure I'll figure out exactly where Genesis fits in with my playstyle once I get some raid experience as a healer, but it definitely feels weird right now (and the mana cost has put me off using it too much due to regen from 85-88 seeming to be an issue since the patch [85-86.5 before the patch, no problem, 86.5-88 after patch have to watch mana carefully]).