I couldn't even get myself to use it in between massive attacks on heroic Spirit Kings. The ability just isn't good.
Yes I use mushrooms as often as possible. Any heal is better than no heal! It's a very cheap heal and can crit for well over 20k a mushroom. (based on your SP of course) With practice, good timing and a bit of skill this can easily be one of your top 5 heals. Works exellent in 25mans due to it healing all players within their range. Druids roll 80% hots so it's never a waste to drop mushrooms while your hots are out. Most fights will have some sort of stack phase where they can be pre placed and ready to use. Some fights do consist of RNG so it's a matter of good timing, skill and touch of luck. :)