This is my current assessment of the revealed megas. It's loosely a tier list, but really I just have different critiques of the different mons and didn't want to upload 9 separate images.
Dragonite, Chesnaught, Greninja: I like this group of megas the most so far in the sense that I think these designs did enough to enhance the original without overdoing it. Of these I actually think Chesnaught is the most successful, but I don't care for regular Chesnaught so it won't make an in-game team. A lot of people were hoping for Dragonite to become cooler somehow, but I think this design does well as keeping its charm while enhancing its design. Greninja I like a lot -- it's a little bit more minimal, but I think it's effective. I like the water shuriken eye mask a lot, and the color change is nice, and I like that it gets a big shuriken in a different way than Ash greninja, so there is some nod to that obliterated design. It's a change from base to mega akin to Charizard X, one of the original set that I thought was a simply good design.
Raichu Y, Delphox, Victreebell: For the most part I like these designs a lot, they just have a mibor flaw or so. I like Raichu Y's change in color/fur pattern, and the change to the tail and... horns? Ear hair? The flaw is that I wish the "horns" were the ears and they didn't include the pikachu ears. I really like the blue in the eyes, so much so that I wonder if I would like this design as much if the eyes were a different color.
For Delphox, it's all around great, I just don't like the beard. Maybe if the beard had been swept back so it looked more like long fur it would be fine, but as is in the static image I don't care for it.
Victreebell is one of the goofy megas, and while I personally probably wouldn't add it to my team I think it's good to have a few goofy designs. My main gripe with Victreebell is that its gimmick is that it's ballooned up because it's holding in all its excessive poison, so when it attacks it slims back down momentarily and just looks too much like a regular Victreebell. I wish they added a couple more features to distinguish it from regular victreebell during its attacks.
Raichu X: it's fine, I don't like this recent trend of giving mons snaggle tooths, but it's fine. In contrast to other posters, I don't think this would look better with Raichu Y's tail, it would just look too much like regular raichu if they did that.
Hawlucha: overall it's fine, but I think it looks too much like it's just wearing a costume, like Ash's Hawlucha did in the anime. I think this could have been fixed relatively easily by making the shoulder "horns" look like feathers so that it would look more like this is an extra layer of feathers rather than a suit.
Malamar: I love the bioluminescence and the iridescence, it really adds to the hypnotic powers. I don't mind the change in body shape, it's cool when a few megas have a very different form factor. But, I just do not like the hair. I get that they're going for a bulging brainy look, but it just looks to me like a haircut that looks great on my aunt but terrible on a Pokemon. I would rather its tentacles look more like a back to the future Doc Brown mad scientist hairstyle. But perhaps even more egregious is that mega malamar puts its "hands" behind its head and "hip" thrusts when it attacks. I don't want to see Malamar's magic Mike impression every time it uses a move. It's ironic that mega malamar has the hairstyle of someone who would report it to HR for its attack animation.