I have a combo and, in all honesty, I LOVE IT! I do my construction in a way that doesn't require to change back and forth between the two, so I overlock everything and at the end change to coverstitch. It was a tad complicated at first, because I had to keep looking at the manual to thread the coverstitch part which I don't use as often. But now that that's stored in my brain it takes me at most 5 minutes (I'm being generous, it usually takes a lot less) to change from one to the other. Probably the same amount of time it would take me to switch machines.
I have the Juki MO735 and it's really wonderful, can't recommend it enoug!