Free to play and fun = win/win situation.
Finding parties to quest with is fairly easy, the game pace is fast and doesn't necessitate a great deal of grinding. The combat system is fast paced and fairly simple but enjoyable, requiring your basic FPS type interface.
The game is very solo friendly (at least in the early levels) so you can go it alone if you like. The game runs off of the tried and true Source engine making it very efficient and stable, running well on even older hardware.
The downsides are that the classes are gender locked (2 of the 3 are female and only 1 class is male) so you might end up stuck playing as the opposite of your actual gender. Although the free customization options for your character's appearance are adequate, a lot of nicer ones require you to pay real money to rent or purchase, which is presumably how the game makes money.
All-in-all, being free to download and play makes this game a no-brainer. It's hard to be critical of something that's free. Vindictus is certainly worth trying. If you decide you don't like it, it didn't cost you anything but time to try. Although the time-tested saying "You get what you pay for" tends to be the rule, Vindictus seems to be the exception. It's a title that's on par with the games you pay $50-60 for in stores only it's free (and legally so).