Just beaten Lost Missions, thoughts below:
+Great weapons and combat
+Decent graphics overall
+PDAs are vaguely interesting
-Starting to show its age a bit on the textures front, there's quite a lot of blurry walls etc
-Very linear, I don't normally mind that but at least 70% of the game is just following a set path down corridors with minimal scope for deviation
-Bit too short at 2hrs in length; I know it is just a 'bonus' section but it all ended a bit abruptly (hell section is very short)
-Final boss ridiculously easy, 2 charged BFG shots and it's game over; heck they even put a couple of BFG ammo packs nearby