I've used both the Springer (for a couple of years, off and on) and the Walkydog (just recently). For anyone not used to having a dog attatched to your bicycle, I'd recommend the Springer - it seems to dampen the force of the dog's lunging and pulling better. But it is not very attractive, and the cotter pin can be difficult to get in. Also, it does not fit well on my new hybrid bike, and I had to have it angled back to provide minimal clearance for my heel. That or attach it to the seat post, and risk having the seat twist with the force of a strong lunge ("But there's a dog over *there*!!!")
The Walkydog is much easier to use, and looks really classy, too. The problem with it is that if they machined the brass coupling incorrectly, it will break, like mine did. They are sending the new part, no problems, hopefully the faulty part was a fluke. The Walkydog is supposed to be attached to the seat post, at a 20 degree angle back...and then you have the same problem of the seat twisting with a powerful lunge. (It is *really* hard - for me, at least - to get the seat tightened down enough to prevent this.) I've got it attached to the frame just under the seat post, instead, at just a little more of an angle.
Both of these products position the dog pretty nearly at your side, easy to see with a quick glance down to the side.
I haven't crashed with either one, and I don't think I'm a particularly strong cyclist. I've got three dogs, I ride with them one at a time, and they range from a "polite and doesn't pull much" 55 pounds to an "I wanna chase that (fill in the blank, he's an excitable dog)!!!" 70 pound chow chow mix.