Woot me, I successfully completed Serious Sam on normal level without cheating. I gratefully accept whatever minuscule amount of kudos is still available to someone who completes an eight year old game. According to X-Fire it took me 22 hours of playing time. The in-game clock shows about half that. The difference is due to the number of times I died and had to retrace my steps using quick save.
As might have been expected the last few levels were completely chaotic particularly the second last stage: an arena in which you have to face off hundreds of tough mobs all charging you down. Eventually I had to rocket jump onto a nearby wall to allow me to shoot the mobs from a safer perch. Although this is an exploit the game designers seem to be OK with rocket jumping. They have even included a number of handy launch pads for getting you back into the action but it does feel less satisfactory that facing the foes toe to toe and shooting it out.
After finishing the game I had a quick look at multiplayer and was surprised to see that there are a number of servers still active. I signed up for a co-op game. Although I had no communication with the other players I could imagine having fun in co-op. The number and difficulty level of mobs seems to be increased compared to single player. Unfortunately there was one player who knew every trick and at each new screen he found a way to get to an inaccessible high point either through rocket jumping or some secret route. Once there he could pick off the mobs with impunity while the rest of us ran around as cannon fodder.
Expert players probably know tricks like this for every part of the game but I do think it removes most of the fun. For me Serious Sam will always be about constantly running and constantly shooting as you try to survive against impossible odds.