I for one haven't used it. The store has advantages, in theory, provided they don't lock down the rest of the machine (c.f. Windows RT)...
1) Windows has no package manager, so a lot of freeware software utilities come from some dodgy download site that may bundle malware in the installer.
2) The update mechanisms for popular vendors run an update service in the background, which no one wants clogging up their machine
3) Applications in the store presumably run in a sandbox, which may improve security.
I'm a cheapskate, so I've never *paid* for any app on Google Play, so from the revenue side 30% of $0.00 is still $0.00