What you're carrying (fittings, size, weight etc) and mounting it too can have a huge bearing on what you can fit and where.
I'm using a
Saris Bones Solo and it's fine with adult alloy/carbon road bikes (<12kg) fitted to boot lid on a 2011 Fiesta. I wouldn't want to carry a heavy MTB on there though! We use it in bad weather for 1 side of my other halfs commute - I wouldn't say it was suitable for use every day.
If you're carrying more/heavier bikes or a more permanent fixture I'd
look at tow bar mounted. I really like the
Thule Easyfold if I needed something heavy duty/multiple bikes and going long distance. Although you do see more people with roof mounted these days - as they're generally much cheaper.