I presume you're thinking in terms of a hatchback type rack because cost is an issue? if not, see above comments. I use a Saris bones rack on a nearly daily basis, and like it well enough. I drive about 10 miles, and bike another 12. The ten miles includes about 8 on the freeway. The rack is stable and secure (in terms of holding the bike on the car), and seems to cause relatively little injury to the bike, relative to my old rack. I have to say, I have no issues with the cables -- sure, it squeezes them, but they aren't rigid, or designed to be immoveable. The bigger issue is snagging them when loading and unloading. The one downside relative to my old rack is that the straps buzz in the wind sometimes, though that's usually a sign that I need to tighten them.

I think, however, that it is higher impact (in terms of wear and tear on the bike) than a good hitch rack (I have a cheap one, and it is not much better than the Saris), and much higher impact than either a roof rack or a hitch rack that holds the bike by the wheels.

As to loading: this will depend a little on your car, but you should be able to dead lift your bike possibly above your head, and maneuver it into place. No, the rack might not NEED to be that high, but you may find (depending on your vehicle) that it needs to ride up pretty high on the car, depending on the design of your rear hatch. Or that you prefer it, because you have more clearance when backing into parking spaces (to make it harder for a would be thief to get to the bike... see below).

The significant down side(s): (to my knowledge, these apply to all hatchback type racks).

1) the rack can be removed from the car without tools, and without opening the car. I could probably bend the hooks a little, and make them a little tough to remove, but the straps are easy to remove from the hooks, and readily replaceable, I think.
2) there is no solid way to secure a bike to the rack itself -- I use a U-lock which I use to lock the front wheel to the frame (really to keep it from shaking its head) or occasionally to secure it to the rack, but it is for show/deterrence only, as the rack is removable from the car, and with a little work, the lock can be removed from the rack by disassembling the rack. simplest for a would be thief would be taking the whole rack and bike, and throwing it in the bed of a pick up. This is most important if you expect to need to stop on your way home or such.
3) significant degradation to rear vision: This will vary somewhat by car, but having a bike behind your rear window is going to cause a reduction in what you see behind you. (obviously a top rack doesn't have this issue, and a hitch rack will tend to hold the bikes lower, causing less of an obstruction, in my experience).

Minor annoyances: If you have a rear wiper, it is likely to be obstructed. Your fuel economy will suffer with it in place (this goes for ALL racks!!!), with or without a bike, so plan to remove it when you're driving significant distances without the bike. Also, weather/UV will take a toll, so you may want to remove it to keep it in good shape. It will take up space when not in use -- if this is a concern, there are 'flatter' designs that may be preferable.

I think this hits the major points -- let me know if I've missed anything significant...