I take my kids out (now almost 10 and almost 7) frequently in most conditions and they can lean over the side, change places in the boat and all that without a problem--though dad does prefer to have some notice before they do so. Now, don't get me wrong, the first time they did that it scared all kinds of stuff out of me! But now, I realise that you can lay a guideboat over very far before she'll go over. I've yet to capsize mine but I've deliberately had her over to the side with the gunwale under water and it seems she just firms up there--kind of neat. I've paddled her like that, with the gunwale on the water, kneeling the center seat, as you might in a canoe to make it easier to turn. Check out:
http://www.redcanoes.ca/becky/canoe/solopaddling.html
for a wonderful video illustration of this.
In calm conditions (wind-rippled water, small waves) I can stand up in my guideboat. Again, it seems to help to lay her over on her side a bit when doing so.
I can't compare it to Clint's Drake (lovely boat!) but I can compare it to a Whitehall. Initial stability is higher in a whitehall, but the whitehall is slower with the same rower (me) and feels heavier. Now, I've not had a whitehall out in rough conditions, but in wakes and the like, I prefer the feel of the guideboat. You sit lower in it, like you would in a kayak, so your center of mass is much lower.
It took a little while, but I'm really comfortable out in most conditions, even when my son (7) looks outside at the trees swaying in the breeze and says, "It looks windy dad. Let's go out in the boat and find some waves!"
Two biggest drawbacks for me: in most conditions, you really can't stand up and stretch when you've been rowing for hours; the sheer line, beautiful though it is, is such that, if car-topping, you'll probably want to modify your roof rack so you can see more than inside the end of the boat
Hope this helps!
lyle