Xtreme (standard) is being upgraded from version 2.0 to 3.21 at a cost of about $50. Pro is being upgraded from 3.0 to 3.21, at the same cost of $50 again. Why this price is the same for two relatively differently sized upgrades is debatable, and is perhaps where people's unhappiness really lies. Upgrading can thus be seen as good value for money (in the standard Xtreme case), or pricey (in the case of Pro).

I suspect Pro owners are unhappy because if they didn't upgrade, then the standard Xtreme version would have superior functionality, but upgrading Pro seems less of a good deal for the money - especially if they already own X3D.

Maybe, a small discount for X3D owners who wanted to upgrade Pro would have been swallowed better, as has been suggested on the forums elsewhere.