My 10c after playing for ~50hrs:
(currently @ CH 12 after spending a decent chunk of time grinding in CH 11).
**No significant spoilers, just a mention of a particular Eidolon battle**
The battle system is as good as people have said. Its fast, tactical, and by and large very fun.
There are a few issues I feel holds it back from being the best in the series though.
The big problem I feel is that battle rating is always based on kill speed, which I think kind of undercuts the need for strategy as you always end up favouring characters that are better at damage dealing over those that have higher survivability. For example I always end up picking Fang over Snow as while both are excellent Sentinels, Fang's higher DoT seems to invariably trump Snow's higher HP and broader range of defensive skills.
The lack of formation control (i.e. positioning of party members relative to mobs) is an understandable omission, but its still disappointing as its something that I think would again make for better tactical battling.
The overall scenario is decent, typical Kazushige Nojima high concept stuff, but the way its expressed in-game is I think pretty terrible. Without the copious datalog entries to clarify story and background points I would have been baffled by half of the cut-scenes events.
Dramatically its a mess. Despite some decent voice acting in spots (Vanille is notably and consistently awful though), it just seems to me that its a case of Square bullishly folding in the usual roster of FF franchise cyphers and expecting it to work as a dramatic story.
The fundamental flaw seems to me that while the basic plot and scenario is relentlessly dark and nihilistic, the dialogue, music, and characterization conspires to undercut this tonally at every turn! Its like Schindler's list recast with boy band members and mouseketeers in order to maker it more upbeat and kid-friendly!
On the subject of music, Hamauzu's score is on the whole extremely good and highly listenable on its own terms, but the way its used in the game seems almost random, especially in sequences that should have real emotional resonance.
Its just baffling to me. Looking back to FF7, back then they had the sense for the fight with Jenova following Aeris demise to not use the normal upbeat battle music but with something more somber and fitting to the moment. There's a HUGE reveal in XIII prior to the Hecatoncheir Eidolon battle for example, but then its just a-nother summon battle... it actually takes away from the moment by creating a moderate spike in difficulty that for most players is going to result in them skipping this crucial cut-scene in order to get back and retry the battle asap.
What's especially disappointing is that from a technical and artistic perspective FFXIII is a wonderful achievement. Its a gorgeous looking game that clearly has had an immense amount of work put into it. Personally I don't have an issue with the overall linearity and the lack of traditional towns to explore when every part of the game looks as good as this does.
tl;dr
FFXIII thus far is an astonishingly badly "directed" game.
Pacing, tone, and characterization are all way off, yet the game remains fun and worthwhile due to the sheer quality of the presentation, sky-high production values and most of all the highly engaging battle system.