PowerDVD has a nice interface....but I don't really use it much so I don't care.
I like WinDVDs right click menus better....Fast Forward/Rewind are options on the main menu....with PowerDVD they are submenus.
Also in WinDVD Title is one submenu and Chapter is a different one. So assuming you want the same Title you only have to go one layer in to change Chapter (the whole movie will be one Title, the other Titles are special features and the intro sequence and stuff like that). Whereas in PowerDVD the Chapter is a Submenu under the Title Submenu. So if you want to change Chapters you have to go through the Title first.
Well those are my two biggest peeves about the PowerDVD right click menu....and yes PowerDVD has a much nicer control panel than WinDVD, but I watch DVDs full screen with the control panel hidden and so I care more about the right click menu than the control panel.
Also as Zucchini mentioned, PowerDVD doesn't decode the LFE channel. If you are using AC-3 passthrough this doesn't matter much, but if you are using other sound modes then you can miss some bass. Alot of movies don't make heavy use of the LFE channel, but when they do it's usually very heavy bass. The other 5 channels are full range so you get bass in them two, and in multimedia speakers that's all directed at your subbie anyhow, but the LFE is louder relative to the bass in the other channels. The main reason I prefer DVD over VHS is sound quality, and PowerDVD just isn't up to snuff in that area.
I use WinDVD2000 2.3M and I also have a hardware deocder (this is pretty well exclusively for TV-output becuase my Video card doesn't have it). WinDVD2000 is the best DVD player on the market in my view.
It's possible that PowerDVD is better in Visual quality though I can't tell the difference between WinDVD 2.3, PowerDVD 3.0 and the Hollywood Plus with 1.82 drivers. But for sound quality there is no comparision in the favour of WinDVD 2.3 (the hollywood plus is sort of out of the running there...it has 2 channel output or AC-3/DTS passthrough and you need a decoder, so either it's sound quality is perfect if you have an external decoder or it's horrid with only 2 channel).