As for the instances: some are bound to just you or your group, some are open zones for many groups. I haven't found it to be a problem at all and is actually pretty neat imho. You'll find most of the gripes people have about AoC are simply "that's not how WoW does it."
It is a great game. The graphics blow WoW away, the combat system is innovative and actually fun, and overall there are a lot of subtle details that you will appreciate. Having said that, the game is still very buggy. The last patch especially has seemed to make crashes inevitable every few hours. There are a lot of lag spikes in instances, however, unlike wow, after the server lags, you don't come back to find yourself dead, the game just picks up right from where you left off like it never happened.
Also documentation and tooltips in the game are very poor so far. My recommendation is that if you do start playing, don't judge it too quickly. The lower levels are a combination of boring, easy, and frustrating as you try to figure the game out. However there are 80 levels to go through and it definitely improves. The other gripe my friends have is that in WoW characters always looked totally bad@$$, but with the realistic armor and just the number of armor sets in AoC, the npcs look much cooler than any player you'll see.
I'm assuming you don't have friends who play since you are asking if it is good. If you plan to play on a PvP server and don't have friends to play with, you have to be patient because you will get ganked a lot, but you will also have the opportunity to gank others

Funcom is working hard to balance the game dynamics and improve the interface. I'm having a ton of fun and only expect the game experience to improve with subsequent patches.