This game got a lot of poor reviews since it isn't something that is likely to appeal to the sheeple mainstream, but I absolutely adore it. It is the most fun I've ever had in a 2d sidescrolling shooter, and for speedrunning or just generally screwing around it is a blast (especially once you get the jump pad and master its use). Sure, the premise/story/gunstacking/etc. stuff is silly and over the top, but this is Serious Sam! It's all *about* the insanity, and despite DDXXL being a kind of 'side game' to the FPS trilogy, it is actually one of my favorites.
If you like frantic shooters, tons of secrets to discover, advanced movement-based abilities, completely ridiculous scenarios, a very unique visual style, a decent amount of challenge modes, and just generally blasting the hell out of everything you see... Serious Sam Double D XXL is for you. Screenshots absolutely do not give it justice, and many of the videos I've seen just don't capture how fun it is to *play*.
For the traditional Serious Sam fans: I understand why you wouldn't really be interested DDXXL, but give it a shot as if it's a totally different franchise... I think you'll really enjoy it once you get used to its oddities. Yes, the campaign is fairly short. Yes, some of the challenges are just spam-fests. But it has fun enough gameplay and mechanics that you'll love to replay it over and over.
And for those that like to speedrun, glitch, etc. this game is a treasure trove of awesomeness. Some may consider that a symptom of a buggy/predictable/whatever game, but it's really only like that if you're intentionally trying so I consider it a feature. Have fun breaking it to bits and seeing what kind of insanity you can cook up!
My only real complaint: the unlocked difficulty levels really aren't that much harder since you start with all your guns and upgrades that you got during the previous playthroughs. I really wish they had made the harder difficulties packed to the gills with more enemies and more difficult situations, but unfortunately the first few levels are pretty much exactly the same on harder difficulties as they are on the easiest. This really isn't that big of a deal (as I'm sure many find the game very, very difficult as-is), and more just a wishlist item if devs ever returned to it.