I've always felt that exploration is the entire point of the Metroidvania experience, and Hyper Metroid is probably the most well done exploration hack I've ever seen. It also never becomes ridiculously hard; I had a lot of trouble with Draygon and Ridley, but not enough to sour the hack.
Every area looks very nice. The Wrecked Ship and Maridia are really improved from the original. Tourian wasn't the most amazing area gameplay-wise, but the design was pretty good and the escape sequence was awesome. Crateria, Norfair, and Brinstar were ok. The game had a lot of the original assets, but there was a large quantity of new stuff and none of it looked very out of place. The enemy choice was good and overall I liked pretty much everything in the way of level design and so forth.
If there's any cons... it's nothing really big I can think of. There's a few awkward mandatory speed booster areas, but that's probably just because I suck at using speed booster. This is the first metroid hack I've ever completed, and I enjoyed it from start to finish!