The Unofficial Patch is a combination of the default Basic Patch which mostly fixes bugs only, and the optional Plus Patch, which is kind of a mod adding various tweaks and implementing a lot of unused content, including new quests, levels, weapons, items, characters, dialogue, graphics, models, sounds and original music! Included are also the optional Patch Extras which feature Player Mods, like shaders suites, as well as Developer Tools, like the Unofficial SDK to mod the game.
I never played the game before because I heard of the bugfest it was on release.
In a whim I decided to try it out now as I am quite intrigued with the gameplay and story. Running the game without a patch on a modern PC and OS (win 8.1) is quite impossible without applying a patch. And I must say Mr.Wesp has done a terrific job in making this patch. The game runs smoothly and the graphics look very good for a game this old and I haven't experienced any glitches until now.