To me this is one of if not the most important aspect in a game (which is not how the game actually plays). Consider it this way, do your favourite games have bad or average soundtracks... No they do not, unless you have no opinion on this due to disability (deafness). The soundtrack is often ignored, this is done less these days however the music should be recognised and commended more. If you look at some of my favourite Games they all have fantastic soundtracks: Splinter Cell: Chaos Theory, has an excellent soundtrack done by Electronic artist Amon Tobin (he also did the majority of the InFamous soundtrack), the soundtrack captures the atmosphere of the game perfectly, with a change of tempo if you are discovered, if you havenít heard the soundtrack or remember it fondly it is available to listen to on both Last.fm
and Spotify. Another of my favourite games is Full Throttle, a classic Tim Schafer adventure game by Lucasarts, which featured music by The Gone Jackals, whose music fits perfectly into the Heavy Metal Adventure, despite by todayís standards it is very low quality, which is entirely forgivable due to it being 14 years old, here
is a link to said games intro. My final example would be the Halo series (particularly 1 and 3) the fully orchestrated soundtracks work fantastically, despite what your opinion of those games is you cannot knock the soundtracks, the Halo theme is Iconic and clearly recognisable, it feels right, it feels like Halo; which is the exact thing you want it to be.
Unfortunately music in games is not perfect. Not enough games follow in the steps Lucasarts did with their adventure games, a system called iMuse was an interactive platform that allowed music to be synchronised perfectly with what was on screen, whilst seamlessly transitioning from one piece to another: Monkey Island 2: LeChucks revenge includes the best examples of this. The reason that iMuse wasnít well known was due to how well it worked, generally people will only notice a system such as this if it is done clumsily.
Ideally music in games is unnoticed until you are looking for it; this is not to say you donít hear it. I consider it a success if it fits into the game perfectly; you notice how well it is implemented as well as orchestrated in later playthroughs.