Honestly, I never would have sought out "Serious Sam Double D" on my own. But since the "Serious Sam" bundle was so stupid cheap not too long ago (Oct. '14), I installed it just to see what it was about.
What I found was a glorious hybrid of "Contra" and "Borderlands". The gun stacking mechanic can be as ludicorus as you want... provided you've found enough weapons and connectors, and purchased the mods. Gameplay is not at all complex, though there are a few physics puzzles that will challenge you. Old school gamers who know when to run left, jumper high, or look for hidden paths will be rewarded.
"Double D" is even more over the top than the regular Sam games, and is genuinely funny. Especially on a second playthrough after you unlock Huff. After a few hours of Sam Stone's ultra-serious machoism, you get to experience the whole thing again with Huff's ultra-redneck belligerance. Really, the plot is non-sensical and non-existant. The characters are cartoonish, and the worlds full of in-jokes and Internet memes. So yeah, a lot of "Borderlands" flavor, only you're not stuck with just one gun at a time.
I highly recommend a second playthrough with Huff on higher difficulty. You'll have all the mechanics down pat (and all your upgrades), and will need to be challenged. Plus, Huff's dialogue and cutscenes are great fun, even if you're blasting the same enemies. There're just more of them.
Controller support is just fine, so it makes a great pick-up and play game. The graphics have a hand-painted look that hold up very well in HD. I haven't tried local co-op, but it's a shame there's no online feature. Not a dealbreaker by any means, though. Good value for under $10, a steal at anything less. Don't hesitate to pick it up on sale for yourself or gift to a friend. Seriously!