Generally the lighter the camera, the more stable it will be with a minimum of support. Heavy cameras tend to need strong cheeseplate rigs that have multiple supports to brace from. They also need to be fairly level and so some extreme angles are a bit more tricky to achieve. But this cheeseplate from Filmtools has a full sized ball leveling head on it that is easy and quick to change the camera angle.
The biggest problem you need to combat when mounting a camera is bounce. You really need to try to brace the rig so that it is solid and does not have any movement in the camera and ultimately the image.
Suction cups always need a fairly flat and clean surface. Unfortunately, modern cars have a lot more curves and less flat surfaces, therefore, it is very useful to have a variety of differing suction cup sizes (4", 6" and 8" cups). This way you can now mount off small flat surfaces like the sills of the car or a spoiler and this opens up a greater possibility for camera positions and stability.
For a strong and solid mount, try not to brace off the bumpers of the car. Bumpers these days tend to be plastic and naturally are designed to have movement in them. In the past I have braced a camera off a bumper and as soon as the car hit a bump in the road, the camera wobbled violently and made the image unusable. It is OK to use bumpers for additional support to the main brace, but don't use as you main fix points.
I have been using the Canon 7D to film most of the car work as it is small, light and very flexible to put in most positions. Using the 7D is great, but if you don't mount a solid grip then you will get the 'jello effect' that has been so widely documented. This is even more important if you are mounting a bonnet camera because as soon as you start the engine, you have a lot of vibration that transcends into the image sensor of the camera and that can completely ruin the footage. Its another reason that I favour the 7D over the 5D mark 2 for this type of work. The 5D has a slightly larger image sensor and therefore is slightly more unstable when it comes to vibration in the sensor - and these cameras are subject to quite a bit of turbulence and vibration when driving at speed off the side of a car and an uneven surface.
Recently I mounted a bonnet cam on a chase car in order to film the new Fabia Vrs from the rear. I had always favoured a tripod mount that triangulates a solid footprint, because I had always thought that this was the best and most secure way to rig the camera.
However, I suddenly decided to try something different. I took a fourth fix from the car windscreen to the hotshoe of the top of the camera. This four point diamond shape really made the camera as solid as it has ever been. I could quite literally move the whole suspension of the car by just trying to move the camera...it was that strong and solid.
It also helped to run a long HDMI cable from the camera to the monitor in the cabin. This way you can see when the subject is in frame and in focus as we didn't have a remote follow focus on the rig. In my experience, this is without doubt the best way to mount a camera on the front of a car. I have not yet tried to mount this diamond footprint in any other positions and it could be quite difficult to do so without the angle of another surface, but I will try to replicate this again and see how it goes.
For suction kits, Filmtools.com has superior mounting equipment and pretty much everything you will ever need. However, the shipping costs can be high as the equipment tends to be fairly heavy, so always check first. A good UK based alternative is Cameragrip.co.uk , but their range is a little more limited and not as adaptable.