Inn has a lot of potential to be great, but it can also be pretty poor.
Inn is one of a small number of cards that you play a top deck control game. Like KC-Scheme, inn can set things up so you can green at will and just power out the game without worrying about future shuffle luck. For instance, if you have enough cheap actions and 2 counting houses you play a turn of cheap +action/+buy (pawn, hamlet, etc.), CH (draw all copper), and then buy an inn and either a province or more coppers. Once you have only actions in your draw deck, you can reliably drop a hand of counting house, +buy, and +action back into the discard. Clean up returns your actions to the discard and your coppers go back to be drawn as well (including any you purchased this turn). You can then play counting house, your +action/+buys, and buy another inn and a province. If you have time to set up reliable decks like these, inn is an insanely strong enabling card until you run out of them. If you want a reliable Inn top deck engine you either need to duplicate your engine so one is in the discard/draw and one in hand at the start of each turn or gain an inn during your turn (university, IW/highway, remodel/any three, upgrade/any 4, etc.) and then play draw to churn through new action rich deck. Things that can force your deck to go into the discard are also helpful: chancellor, loan/venture (made even better by herbalist), golem, and hunting party can all dump your deck and setup an every other turn inn top deck engine.
Alternatively, you may just need to play the odds. For some setups, like KC/witch/bridge, the game may literally end up being a race to who gets inn at the key moment in the game (well, I'll just shuffle up a deck of KC/KC/KC/Inn/witch/bridge/bridge/bridge now that I bought an inn with all my actions in the discard). So yeah, I will sometimes play huge megaturn "engines", just trying to buy an inn at the right moment and then setup a game winning turn. The usual suspects for megaturns hence complement inn quite well - KC, bridge, highway, big draw, etc.