I recently bought it and FF 13-2 cheap, after ages of hearing all the OMG TERRIBAD! BURN THEM! My thoughts after having recently beat it. It's not that great, but it certainly isn't the worst FF game i've played. Can't speak to 13-2 as I haven't started it yet.
I will also admit that while I really don't care one way or another for any of the characters in the 13 series (apart from Sazh who I kinda like) I am cautiously optimistic about 13-3 as the complete departure from the current gameplay system looks good.
Anyway, I'll my thoughts on 13
Story/Characters: Yeah it makes sense if you do all the reading on it (which you shouldn't have to do) but it's still ridiculous, even by FF standards. The narrative is a mess, a lot of the characters are either unlikeable or just not developed enough outside boring cliches to care about. They feel a lot like cardboard cutout versions of other FF characters. When the story ended I honestly thought there was supposed to be something after, as the game did not really feel all that climactic or epic to me. Not saying it needs to be, but I felt completely underwhelmed.
Art Design/Graphics: Graphically the game can be quite pretty, and the cutscenes were some of the best looking I've seen. The art direction was hit and miss, some parts looked great (particularly character designs) but the environments were rather bland or empty. Monster designs actually were the worst part for me as most of them were just palette swaps of monsters from one dungeon prior. The Eidolons were kinda cool though.
Music: Honestly, I barely remember any of the music from this game which for a FF game is a travesty for me. This is usually the part I have the most praise for of any FF game, to the point where I own quite a few of them. This was really disappointing.
Gameplay: Now here is the thing, I don't hate the gameplay but I really don't like it either. Once you get to that 20 hour or so mark then yes, the game does become a bit more open with using the paradigm system more. This only serves to make it slightly better than having barely any participation in the battle. Because of the nature of the system you have to work fast to scroll through the menus which leaves you far too open. I also call bull**** on the way you path during battles. You can't control your movement directly but your characters don't remain placed in static positions all the time. So you can get hit by an attack that your character will stupidly walk into on its own accord. The level up system was kinda just there, I didn't like it but I've seen worse. The Weapon and Accessory system was dumb and poorly designed. It was kinda nice that there was no best weapon, it all depends on what property you value in your weapon. However since you are encouraged to upgrade your weapons, you might find yourself leveling up a weapon and upon getting a new one don't want to use it because you've wasted so many resources leveling this one. On the whole it certainly isn't the worst gameplay I've dealt with in an FF game (12 still remains the worst imo) but it wasn't engaging.
FF10 was the game I had the most fun with in terms of combat, though I disliked the linear approach to environment and level design. 7 had a nice complex system that allowed for better customization of party members, that could be grindy at times but was very rewarding. I will also say I got a kick out of FF8's draw/gf system if only for how utterly broken it was and how easy it was to become practically godlike before the end of the first disc (enemies scale with you but you only need 1000xp to level up for every level, never using magic and junctioning high level spells making you nigh invulnerable, and of course triple triad and card mod to get high level gear before even doing your first proper quest). Compared to those and most other FF titles, 13 really was boring at best and rubbish at worst.
Really though I would say the game is subpar but I don't properly hate it. I dislike it compared to others (both in the series and rpgs in general) but I can't muster actual hatred for it like I can with FF12. (And no, I don't hate 12 because Balthier wasn't the star and Vaan somehow wrecked the whole game for it. I hate it because there wasn't a single thing about it I liked or found fun).
For clarification purposes, of the FF games I've played: 6 > 7 > 10 = 9 > 5 > 8 > 12(which ranks highly on my all-time worst games I have personally played list and I will not stop harping on it).