There are many things I liked about Dark Messiah but the lack of RPG, On Rails story and bugs caused the game to suffer greatly. I just finished a couple of games and made some 5 mods that made the game much better, not perfect by any means.
Anyone remember the Flies bug, once it was activated as a sound file (fresh kills) it would go on and on, seemingly forever. There even seemed to be a bug using the Lightning Daggers that made Fly sounds, heh. This mod fixes this issue by adding a Blank sound file in it's place.
Kill the Flies (
http://www.gamefront.com/files/20062223/Kill_the_Flies)
The Nightvison spell given for free at the beginning of the game and uses no Mana was excessively bright. It caused Whiteout of almost everything, especially near Light Sources or White Spider Webs. The Spell Wizard Eye is now mostly fixed, I say mostly because I was only able to remove about 90% of the Whiteout. Now you can actually see objects and floors with Almost Colors, the only Whiteout left are the Light Sources and Spider Webs but they have been reduced by about 70%.
Wizard Eye Fix (
http://www.gamefront.com/files/20062224/Wizard_Eye_Fix_zip)
A fairly minor issue but still extremely annoying to a Stealth player was the Extremely LOUD sound effect of Crouch/Stealth/Sneaking on Metal. It was so loud as to be close to running sound level, now it's much better.
Crouch on Metal Sound Fix (
http://www.gamefront.com/files/20062212/Crouch_on_Metal_Sound_Fix)
There was never a Key Setting to allow Crouch Lock, so we Stealthy types were required to constantly hold down the key. Now if you don't mind typing "exec crouchlock" each time you play, you can have Crouch Lock. ;)
Crouch Lock (
http://www.gamefront.com/files/20062293/Crouch_Lock_zip)
This was a EXTREMELY necessary mod "Xana STFU" in case you don't remember how annoying she is, I won't recount it here as I would never wish to cause anyone that much pain and suffering even in a post. This mod doesn't take away any Story aspect of Xana, it just removes the Utterly Annoying comments like "Wonder if that bow we found could help here" or "hurry climb that ladder" or "the ghoul is getting away" or "this is a dead end". Xana has about 380 sound files, I removed 72 of them and I can't tell you what a Damn Relief it is, she still annoying but much more tolerable. The way I figure it is she needs to still have her attitude come through due to her Species. I couldn't see removing all the annoying parts, then she would come off as nice, which clearly isn't the intent of her character.
Xana STFU (
http://www.gamefront.com/files/20062227/Xana_STFU_zip)
Also Gamefront's Dark Messiah (
http://darkmessiah.filefront.com/files/Dark_Messiah_of_Might_and_Magic/Mods;8540) has several mods, though the site is closed now and I couldn't add mine to the site.
Good luck