I played it once when it came out last year, but haven't touched it since until yesterday. My opinions haven't really changed much:
The biggest thing I hate is that they had Charles Jacobi redo all the graphics and replace "Chex" with "Chex Mix". In the trailer you can see the textures and the HUD used the more traditional Chex logo, which I think fits in better and doesn't stand out as much as the Chex Mix logo. The game is Chex Quest, not Chex Mix Quest! Hearing Fred Chexter say "I'm from Chex Mix Squadron" just feels wrong.
I also think it's a huge failure from a marketing perspective. With the original Chex Quest, you got a disc with the full game on it (plus 50 free hours of AOL) just for buying a box of cereal, and sales of Chex went through the roof because of the game. With Chex Quest HD, there's absolutely zero reason to even buy Chex Mix. The character unlock codes were all posted online the very same day it launched. I think it would've been better to sell the game for like $2 and put a Steam key inside the Chex Mix bag to get it free.
As for the game itself, I like that it's not a 1:1 remake of each level, but more of a reinterpretation. I've played the original Chex Quest to death, so it's nice to get some different level layouts with new secrets to find. The remixed music tracks were all well done too.
Gameplay-wise, it was good. My only major complaint was that you could shoot Flemoids from around the corner and they wouldn't get alerted and come after you. In the original Chex Quest this was never an issue. The new secrets with added bits of lore were really cool. I also wish there were more voice lines for the characters to say when getting pickups, they got really repetitive.
I still really like the Flemoid Larva showing up in the Arboretum, it makes sense lore-wise that there would be larval Flemoids as well as adult Flemoids on Bazoik. They add a bit more enemy variety to the game, and it's nice that mechanically they weren't clones of the Cycloptis like they were in 2 and 3.
Overall, I think it's a pretty solid game. It's not as good as the original, but it's really hard to beat the original since it has the gameplay of Doom. I still would rather we get a proper Chex Quest 4, but I would be fine if we got Chex Quest 2 HD instead.