I love Earthworm Jim and I enjoyed EWJ 3D, but it's hard for me to be gentle to this game, since it certainly hasn't been gentle to me.
It's still a Jim game, so you can expect it to be anywhere between funny and hilarious plus the level design is freaky, which is a big plus. The content is nice, but the gameplay is a different story altogether.
Sometimes it feels like playing an MMO, as in "run from A to B, do repetitive task X for the umpteenth time, then run back from B to A". The camera movement and steering take getting used to and there's plenty of levels (as opposed to one per old EWJ game) which don't allow the slightest mistake and punish the player horribly if they do make one.
Unless you've got some leet madskillz I'd recommend using a trainer for some of the levels - without one I would probably have a heart attack instead of enjoying the game for what it is - another fun-packed episode of ol' Jim's adventures.
Overall a good, if demanding, download. :)