Hmm tough one. I appreciate that combos can end the game. I've sat through a lot of painful stalemate games I just wished would end.
However, for me, it depends on the combo and the consistency with which the pilot of the deck finds and assembles the combo. Most decks probably have some sort of combo - some, are designed for a quick one-trick knockout of one troublesome opponent, some are designed to hit the whole table and end the game etc. I don't mind any of them in principle, but, I personally don't find a deck that does the same combo every game, every time, every win very fun to play against. That's just me. I don't run many tutors (usually none, if you ignore things like
Evolving Wilds - people ignore that this is, actually, a tutor), and I rely on redundancy and general overall synergy for consistency.
Some of my friends in my playgroup run certain combos that annoy me more than others - mostly because of the answers my own decks can, or cannot, have to those combos. So I guess it's not the combo, but rather my lack of ability to counter a particular combo that makes it less fun for me.
What I will say, is that if you run a combo, please, please, please, make it quick. I like pilots who get their pieces out, say, ok, so let me run you through the loop, explain the combo, ask for responses and that's it if we all agree the combo goes off. Slow, annoying combos that take forever but are inevitable drive me nuts (again, I will refer to my earlier point that this is in part due to MY lack of ability to do anything about it that makes it frustrating). I find it interesting that practically no one who dislikes combos seems to acknowledge this.