Final Fantasy XI is a very enjoyable and content-filled game, with a bit of a steep learning curve. Due to the game being out for such a long amount of time there is an insane amount of content to experience and to play through, and all of this can be exteremly overwhelming to a new player, especially if they don't know what they are getting into beforehand. Some of the more recent updates and patches make it easier for new players to advance and get caught up, but you will definitely need to put in a very large amount of hours to really even begin to scratch the surface of what this game has to offer. I'd reccomend reading guides as you play so that you are better able to understand what you are doing as you play, and what you should be working toward.
Gameplay wise I found the game to be extremely enjoyable, the battle system is a bit slow to start out but becomes more intricate and interesting as you advance with the introduction of skill chains and its very party-centric battle system. That is one of the main features of this game, the almost necesarry party interaction that is required to advance. This was particularly true early on in the game's life-cycle, but it has become considerably easier to solo with the introduction of some new jobs, the tuning of enemy strength, and especially the recent trust system that has come about. The trust system is basically the ability to build a party with three npc party members, making it much more accessible for new players to advance, and making it more possible for players to solo in general.
One of the interesting mechanics that I most enjoyed in this game is the job system that it has in place. Similar to Final Fantasy III or V, you are able to assign your character to a job/class and change that job as many times as you'd like. This allows a player to experience everything that the game has to offer on one character rather than having to make alternate character to play with.
Community wise the game may seem a little bit dead or slow if you're just starting out, as most players spend their time in the end game zones, so the starting cities and anywhere else may be a bit empty compared to other MMOs. In my experience it wasn't too noticeable however, there were still people running around Bastok (one of the starting cities) and plenty of people sitting outside of the auction house at any given time. I would reccomend looking into the more active servers before playing though, doing a google search for Final Fantasy XI server population would probably help you make a decision if you're looking for activity. However, I'd personally reccomend trying to find a linkshell or a group of people to play with before starting the game, and then joining a server where they are to get the most out of the game. This game, like many MMOs, is a lot more fun to play with other people, and it is a lot easier to make friends as a beginner outside of the game than it is in-game due to how far most older players have progressed.
Overall I'd recommend the game despiting it looking a bit dated, the gameplay is extrememly enjoyable and the community is pretty great and mature as well.