As someone who bought MM-1 off the shelf way back in the day, and have played every game in the series other than 9, the mixed reviews made me hesitant to jump in. But picking up the deluxe version (includes MM6) on sale I jumped in, and have been pleasantly surprised.
Reading other reviews, I can relate to a lot of the pros/cons cited, but for me things other people found as cons (combat, grid movement) were very much pros for me. I think what this game does well, it does very well, but it may not be to everyone's taste.
MMX strips out a number of features that existed in 6-8 in particular, but ultimately that makes MMX much more of a tactical game than its predecessors. In those games in most environments you could easily exploit the strength of ranged combat and the opportunity to toggle in/out of turn-based combat to kite. I may have, at times, even exploited running out of dungeons to avoid a fight or having to reload in those earlier MM games - that doesn't work here.
Kiting in this game is only at all possible with certain class combinations, and the 'ambush' nature of many encounters and the inability to move with an enemy already in melee range make that impossible. MMX will require you to rest, and use potions, and buff/debuff. If you open a door in a dungeon without saving or resting first, you'll probably regret it. And AFAIK you can't leave an area while in combat to exploit out of the fight.
As for character creation, I'd agree that the options for any individual character are not deep (which may hamper your RP enjoyment), but party selection itself is imo a moderately complex tactical game. The variety of options for party composition are deep. 12 classes and 4 races to choose from, and you're almost certain to have to forgo something you really want in your party (whether a race, or a buff, or dps combos).
Puzzles are pretty good on the whole, story is meh (but personally I don't think plot has ever been high on the list of reasons to play MM), and the skill system is true to its predecessors while offering a variety of small tweaks/improvements.
As for being one-shot, that can happen, and generally for 2 reasons: 1) you wandered into an area too high for your party level, or 2) you really needed to get your buffs on.
If you aren't completely turned off by the notion of a turn-based, tactical RPG where there's more focus on the complexity of party creation and skill progress, for me that has been a real steal, and makes me very willing to consider paying full price if they release a follow up. Will you have to save/reload a lot even on normal difficulty? Yes, but for me that's been part of the charm - progression in this game feels like an actual achievement.
Edit - wrote the initial review at the end of Act 2, roughly lvl 20. On "normal" difficulty, this is around the point where the difficulty level swings quite a bit, since you start to GM in skills and have access to the gold and trainers needed to do it. But all in all, I definitely enjoyed it, and after moving through some other games in my backed-up library, will consider playing again on heroic difficulty.