Whenever any game magazine or website makes a list of the best RPGs ever, this game will *always* be on it (along with Fallout, Baldur's Gate, and Betrayal at Krondor).
At first glance, the game looks like your typical BIS-Bioware RPG, powered by the Aurora engine, but this game has a soul of its own that makes it stand out from the rest!
Like many of the previous reviewers have stated, there are a lot of fedex-type sidequests, but most of them have been given a humorous twist that makes them interesting ... for example, trying to deliver a box that nobody wants.
What I liked best about this game was the fact that it's more story-oriented than battle-oriented ... I was never a fan of the BG-style pause-unpause fighting, but in this game I actually enjoyed the battles and was very much challenged by them. There is a lot of conversation going on ... lots of reading ... so this is not the type of the game where you just click "Continue" at the text just so you can hurry to the next battle.
Like I said before, this game deserves a spot in the Best RPGs Of All Time list. Sigil is an amazing place ... you owe it to yourself to discover its secrets!