It depends on what you're trying to launch. It's kind of looking like the next big thing in the launch market is going to be a lot of small satellites in low orbit. This is a market that Stratolaunch might perform well in. It's not the right approach for manned space exploration and probably not for unmanned deep-space either, however.

I'd personally like to see Stratolaunch succeed, but without a dedicated rocket designed to exploit its scale, that's going to be hard. I think Rocket Lab's Electron rocket (a small rocket and a relatively small payload fraction on top of that) would likely benefit greatly from air-launch. On the other hand launching one from the Stratolaunch would be complete overkill, unless you stuck 5 or 6 of them on a rotary launcher or something. An aircraft like Virgin Galactic's White Knight Two would be a much better match for Electron.