I am a professional artist and have been using Photoshop for over ten years. One of my most used features is the "edit" "fade gaussian blur" or "fade sepia", depending on the effect you have used. It works on any effect you apply in Photoshop.
This fade option has allowed me a great deal of control over my images and artwork. For instance, if I add a sepia tint I can fade the tint to whatever level I want after the tint has been applied. That way I have exact control over how it looks. I can experiment while I work rather than only being able to add a certain amount in the beginning. Even using sliders when you add the effect aren't as powerful as this fade feature, in my opinion.
I wish pixlr had this option and suggest it might be a good one to add. I've never seen it in any other photo editing software. It is one of the main reasons I use Photoshop.
I wanted to add that I like Pixlr. The filter effects are very nice.