The buttons are very hard to push. It's not really an issue to me as I generally put it to high and keep it there. I have a Mimi which had a similar (but not quite as bad) issue with the buttons. After a while, they broke in and now they are much easier to use. I would assume the same will be true of Uma. Mine have already gotten a little easier, though I haven't gotten to use it as much as I would like.
The pattern button is a pain because you have to sort of press it in and it isn't raised or anything. So you almost have to have a nail to do it. I don't have nails and I have a bit of trouble with it sometimes. I don't use patterns though, so this doesn't present a problem for me personally.
If you're the type that will change patterns and speeds a lot when you use something, it may be an issue. If not, I wouldn't really be too concerned about it. It's a personal thing though. Since I don't fiddle with buttons, I don't really worry too much about how hard to press they are outside the five seconds it takes to turn it on and get it to the speed I want.
G-Ki is not nearly as strong. It is a full two vrooms lower in strength at the internal part. I love G-Ki, but it is weak internally.
I don't have Form 6 to make comment.
The Vr8 or Vr9 are both good, but different. They are a different type of rumble. They are a hair weaker than Uma, but still very strong. They don't have any patterns, so if you like those that may be a problem. I like my Jopen's better than Uma, personally. The vibration type (the specific rumble of the Jopen vibrators) works well for my body. Uma is wonderful, too.