Puzzle solving using character-driven battle strategy; does exactly what it sets out to do. Love the online play :-)
Online mode quickly became addictive. To fully enjoy it, I strongly recommend completing the quest first. You will gain characters and artifacts you need, as well as learn valuable strategies.
While I've heard others say that there are "unbeatable" characters in online mode, I have not experienced this. Each character has very strong strategies that can be used against others. If you discover a character with a strategy you can't beat, start playing that character until you understand the strategy. Eventually you'll discover the strategy's weaknesses, and characters able to beat them. Many complain about Varkas and Nadia, but I've been able to beat both with different characters, as well as with themselves.
The add-on pack of the "boss" characters is fun, but not necessary to enjoy online mode. I've been able to beat all the bosses using the character that beat them in quest mode.
Reason this isn't a 10: occasionally (rarely) there are glitches in online play which result in a disconnect instead of a win. This has only happened to me twice in over 900 games. Also, some players seem able to maintain a streak even when they lose (in other words, I get a victory counted, they get a defeat counted, but they keep their streak of wins anyway). This is not connected to any online or quest artifacts that I can find, and seems a bit unfair if you care about ranking. It does not hurt me as a player, since I can still win.
If you just want to have a little competitive fun, and like solving puzzles, I strongly recommend Might and Magic: Clash of Heroes.