I didn't really have a problem with it.
Yes, there was a noticable spike in difficulty once I got to Gran Pulse. I'd been running mostly with a Commando / Ravager setup until then, and when I tried that on the Pulse denizens, I got my arse handed back to me. But I found that if I swapped my paradigms a bit and used a Sentinel to soak up the damage (which I hadn't needed till then), things got easier. Wasn't long before I could take on the bigger guys.
XIII isn't my favourite game in the franchise, but neither is it horrible. I won't deny it's linear, but then so is something like Uncharted - it's because it needs to do that to tell you the story. I
prefer something that feels more open, but I'm not going to ***** and complain just because this iteration did it differently.
This is the thing we have to remember about Final Fantasy. They change it with pretty much every game. The battle systems, the magic, classes, jobs, story... Some of them you'll like (in my case, Materia from VII) whereas some will rub you the wrong way (drawing magic in VIII was my bugbear). We all have different tastes; just because a game (any game, tbh) doesn't cater to your particular likes, doesn't mean the series is "over" or "ruined".
Sorry, I'll get off my soapbox now.