Just completed this, thoughts below:
+True to its roots, lots of mindless carnage with some levels having over 1000 baddies. Better than recent 'imitations' such as Hard Reset in that regard.
+Very good configuration options
+Good visuals
+Some epic fights towards the end, often I had to resort to sticking close to a rocket crate and just spam against the onrushing hordes then refill
+Game length felt about right for a cheap action-packed game, ~12hrs
+I like the gun turret thingies, gives you a few tactical options to draw enemies into their fire
+Movement speed is improved which helps for big open games like this
+While they can be frustrating to kill I thought the space monkeys were very well done, the way they leap around between the scenery is excellent
-Lame game mechanic of making you lose all your weapons on level 10 so you have to go around collecting them again. Kinda sucks when you are in a massive open desert level but don't have any good long range weapons like Minigun/Devastator/Cannon and limited assault rifle ammo. What make it even more annoying was that at the end of the previous level I'd been preserving key ammo (minigun, devastator) by mostly using rockets and topping up from a crate
-Indoor sections are very dull and similar, basically running around in the dark looking for switches. The earlier games handled those sections better.
-Early levels are a bit slow to give you weapons and I'm not sure the grunts fit with the game style
-Performance can be a bit suspect in places and not always where you'd expect, e.g. the first power conduit I got to (indoors, no monsters present) it dropped to around 35fps compared to 50-100fps in most of the game. Also some strange situations where fps were under my cap of 120, yet GPU usage and the highest cpu core usage were both only around 70%, with VRAM and RAM to spare. So no bottleneck I could see yet framerate wasn't what it should have been?!? Bizarre
That said I shouldn't be too critical given the way it handles big fights that would bring most engines to their knees.
-Pretty heavy on VRAM, on my 1.25GB card I turned it down to High to keep within the limit
-As mentioned the secrets aren't up to standard, in the original you'd usually be rewarded with some sort of joke or massive explosion (I felt so rewarded trekking round a massive pyramid to find a detonator that blew the whole thing up LOL!)
-Much less scope for trekking off-piste, well annoying when you are in the desert backing away from monsters and get insta-killed without warning by the sandwhale thingy for being too far off the beaten track
-I missed a few of the weapons from earlier games such as flamethrower, lascannon (apparently it is a secret to be fair) and sniper rifle
-Agreed that Kleers over overpowered. In earlier games they usually landed much closer after a dodged leap, so you could quickly close the gap and 1-shot them with the coachgun. Still possible but much harder, I think not having weapons like flamethrower/lascannon also limits some of the options for dealing with them.
More negatives than positives, but that doesn't reflect my overall view of the game. Well worth checking out if you are a fan of the genre.
Pretty much the only thing I dislike about this is that it isn't as candy coloured as the originals.
If you go in the menu there is a Color section that lets you tweak brightness, gamma, contrast, saturation etc with a preview. I found that increasing the saturation made it feel a bit more vibrant and closer to the original, I also lowered the contrast and increased gamma a tad which made darker sections a bit brighter.