According to most sources the current release candidate is not significantly
different than the final version will be.
Who knows.
In my experience the biggest problem with Win7 RC is driver issues,
particularly chipset drivers, as usual AMD more than Intel. By and large
most Vista 32 or 64 drivers will work with their Win7 counterparts, but by
and large is not 100%. Some drivers are indeed compatible but balk at being
installed on the "wrong" OS. When this happens Win7 usually offers to
reinstall the driver and makes whatever changes are necessary to get the
driver installed.
For a Microsoft product that kind of thing is utterly amazing: it actually
works. I congratulate Microsoft for finally getting something right.
I believe these kinds of installation issues are the biggest problems that
need to be ironed out in the final release of Win7. These are annoyances to
experienced users but deal-killers for most users.
While it is always advisable to do a fresh install of a new OS Win7 is
fundamentally Vista SP3 and I doubt there will be many problems with an
upgrade in place from the Win7 RC to the final version, let alone an upgrade
in place from Vista.
While in the past most users got a new OS in a new computer the economy is
such that Win7 upgrade sales, if Microsoft can restrain itself on pricing,
may surprise a lot of people. Microsoft actually has a very good product in
Win7 if they don't, as usual, screw the marketing pooch.