Arcanum is great. If you like roleplaying games at all, you should definitely give it a try, especially at this price. Everyone has mentioned how awesome the setting is, but it does bear repeating. Take a standard Tolkien-esque fantasy setting, complete with magic, elves, dwarves and orcs, but then add steam engines, trains, guns, and the like. Add to that the fact that the new technology is diametrically opposed to magic, so technology doesn't really work well on magical people and magic doesn't work well on technological people, and you have a really interesting place to roleplay. The character creation system is fantastic as well, and it really lets you build any type of character you can think of. It's like the Fallout character creation system, but better, with more options and possible builds to use. The additional of different races makes things interesting because some races have specific aptitudes towards magic or technology, and there is quite a bit of racial prejudice in the game as well. There are a huge number of technological skills to learn and even more magic spells, so you can put together something truly unique.
Like Fallout, one can play through the game in a variety of different ways. You can be a hero or a villain, magic user or techology user or both, a sneaky thief or a burly fighter, or simply sweet-talk everyone and recruit tons of AI companions. The game itself feels a little less "open" than Fallout... while you can technically do whatever you want, there is definitely a more overarching plot to Arcanum, and following it reveals new places to go and things to learn. The player may feel forced to follow the main plot, but it is excellent, with all sorts of great twists and really interesting developments. Also, while following the plot there are many different ways to tackle most problems. The one exception might be dungeons... usually the only way through these is to kill things. There are lots of different ways you can kill things, and some sneaking might work too (I actually haven't tried a sneaky character yet) but it's not really possible to sweet-talk your way through these areas (but you CAN sweet-talk through lots of other things in the game). One early dungeon especially is quite aggravating to get through, but stick it out! The game is totally worth it.
(By the way, it IS technically possible to go through the game without killing anything, but you will have to knock creatures and opponents unconscious instead. I haven't tried it, but I imagine it would be quite a bit of trouble.)
Combat is, unfortunately, less good than in Fallout. The inclusion of the real-time combat seems to basically be just to enable online play, which as far as I can tell no one really ever used. Turn-based is OK, but there aren't as many tactical options as in Fallout, and unfortunately if you have AI companions, they are not very smart and will not know when to hold back instead of rushing into fights. It's still interesting enough though and more of a minor complaint than anything else.
The other big gripe most people have about the game has to do with game balance. I have only tried two characters so far... a straight melee fighter and a technological character who uses grenades. The melee character was very powerful, such that nothing in the last half of the game really posed much of a challenge. Things got a little boring with this character after a while. The grenade user had a little bit of a tougher time, but was also mostly OK. I've heard that guns are actually quite a bit tougher, not doing a whole lot of damage and with ammo hard to come by. I haven't tried them myself though so I can't say. I've also heard that magic users tend to have a much easier time than technological users. Personally, I don't mind such balance issues as much, because I just enjoy roleplaying the characters, and frankly a bigger challenge would be nice, relative to my melee character.
One more thing that I think is quite cool about technological characters... rather than simply buying technological equipment, most of the stuff you have to build yourself, using starting materials and schematics (and also you need high enough skill). For instance, my grenade user could make molotov cocktails out of fuel and rags found in garbage bins. There are junk dealers around who can sell you springs, scrap metal, tesla coils, etc., all of which can be jury-rigged into equipment ranging from hardened armors to electrical tophats that deflect bullets to motorized daggers to steam-powered automatons that help you in battle. It's a pretty cool mechanic.
This is the type of game that you play again and again, trying out different characters and tackling things differently each time. I've only done two characters but I can already think of five or more other ones that I want to try. If that sounds like your kind of thing, definitely grab this.
Final note: Others have mentioned the unofficial fan patches and stuff, but I just wanted to add that they are top notch. There are lots of bug fixes, some content that was cut from the original game is restored, there are some higher quality town maps, and a patch that lets you set any resolution you want (instead of the default 800x600) which is great for widescreen monitors.