I played quite a bit of GTA IV, played it to the final mission in fact (and yeah, "OMG why do you play game you hate???" well the answer is, me and my friend would play it in the same room and have a laugh about how much we hate it, that's kind of a thing that we do, we read the CHERUB book series for the same reason) and yeah, I do think it's a bad game.
I mean, let me qualify that, I think every game is pretty bad. I think the Mass Effect series is bad, I think Half-Life 2 is okay with a story with a story that's alright, I think Bioshock and Fallout 3 are eh, etc. etc. there are few games which I rate as actually good is what I'm saying.
But man, I didn't even find GTA IV fun (on it's own at least, playing it simultaenously with my bro was fun), and my bar for fun, as opposed to quality, is seriously low. The driving was dull, the open world was empty and pointless, there was very little to do and I ended up getting taxis everywhere. The shooting was generic and dull, the story was meandering and aimless, the writing was... alright actually, nothing to really complain about, the themes of the game were fucked beyond all recognition and it's got the most jarring ludo-narrative dissonance of any game I've ever played.
As the OP stated, the lose states are frustrating and the way it forces you back to the start of the mission is even more frustrating.
Why do I feel it necessary to complain, why can't I just be happy that we have differences in opinion? Because according to Metacritic it's the best game ever. And I think that says a lot about our medium, and I think we need to think about why such a polarising game did so well in terms of reviews.