I've been using Photopaint since it started being included with Corel Draw. I now have Photopaint 10 and I must say I'm quite disappointed that Photoshop 6 has surpassed it.
Photopaint has always had a better UI for many things, it takes fewer steps for many common tasks.
But Photopaint has fallen behind in some pretty basic stuff such as being able to do quick effects to "objects" (the Corel equivalent of Layers).
In Photoshop, I can select a layer and choose blending options and features such as bevel, drop shadow, glows, etc. are all right there.
In Photopaint, by contrast, those features either don't exist or are pathetically poor and require far more steps. Take Beveling, for instance, the Photopaint version of Bevel and emboss is basically broken. It's just so much quicker to apply quick effects to a layer in Photoshop that do what 99% of users would want that it overshadows for me, the nice things in Photopaint's UI.
And Photopaint is rather buggy. Ctrl-Insert, a CUA convention, works but Shift-Insert will cause the Marque feature to go into "Add" mode and only a literal mouse click on the UI will change it back, very annoying.