Being more than a decade old, this gem has aged fairly well. Pick it up if you're a stealth fan.
First of all, the graphics, AI, and much of everything else feels like it's aged a lot. This is, after all, a 1998 game. And while the learning curve is not that bad even when on the highest difficulty, the fact that the levels are so huge and that the map isn't too helpful on pinpointing where you are or the location of any items or enemies, this makes the various levels so difficult and drag on for a long time.
This leads to a lot of backtracking, which could easily make you spend over 2 hours or so on each level, for a total of 36 hours or more (with there being over 18 missions in total).
The great thing about this game is that it introduced hiding in shadows and sneaking up on your enemies, which is still a blast to do. There are some very excellent examples of level design present in the game, particularly The Sword level, with its surreal-like setting. Even the levels with supernatural elements are fun, though this makes them feel less stealthy than they could've been.
When all is said and done, this is still a game you can enjoy and have fun with if you can find some strategy guides online to help out with getting through the game's missions. After playing through each level enough times, you'll probably better memorize the game and be able to do plenty of great speed runs, as has been the case already with many fans, who then post their videos on YouTube. And stealth fans are bound to enjoy it pretty well today.
You can pick it up for $10 or less on GOG.com, and it is well worth your money.