My wife loves this app! All she wants is a simple Klondike solitaire app. She doesn't need 100 different games or fancy animations. Solitaire XL fits the bill. It looks great, it can be scaled up to a huge size, and it works flawlessly.
The only thing that would be nice to have added to Solitaire XL would be a semi-opaque background, similar to some widgets. As it is, you can choose a solid brushed metal background (gak!), or no background at all. A semi-opaque background would be a nice third choice that would enable you to see apps in the background, but would "mute" their appearance so that it would be easier to see the playing cards (which can sometimes get lost in the background clutter).