I beat X over winter break. It took me about 50 hours, and I went straight through, without bothering with many extras. I loved it... I played 7 extensively, but gave up on 8, and only looked briefly at 9 (but 9 did look better than 8). In my opinion, X is the best FF since before 7 sometime.
The best way to describe FFX is 'polished'. The annoying little things from previous FF games (at least 7/8/9) are gone: no irritating minigames, no throwaway characters (everyone is at least somewhat interesting and useful), not too many summon magics (and you're not made too dependent on them), and not too many random encounters (and none in puzzle areas!).
The old class system is gone, replaced by a more flexible one where you buy your abilities with skill ranks - every ability is on a map relative to others, and you move your characters from node to node acquiring new abilities. Some abilities are "closer" for some characters, and so end up costing less, so that some are naturally tending toward white magic, others toward stealing, others toward high physical abilities, and so on, though anyone can eventually learn any skill.
The story is quite linear, well told, with a few interesting twists. The game's pace alternates between long periods with no random encounters and shorter ones that have them. You usually control how much time you spend in various areas, so you can spend as much time as you like exploring (for instance there's an arena you can capture monsters for and win prizes in, which I didn't bother with).
I really liked the music, though I can't really say why.
The one thing that bugged me a bit was the voice acting (I played the english dub version, which is the one commonly available in the 'States). The story sequences are all voice acted, with optional subtitles. It grew on me eventually, until I liked that of all but one character (that was unfortunately Yuna, the most central female character).
All in all, a beautiful game that I enjoyed very much. Word around the 'net seems to be that it's a big step up from Square's most recent efforts, and I agree wholeheartedly.