The buttons for the spoiler tag do not appear in the WYSIWYG editor. So you are forced to enter manual [spoiler] type tags.
Further, the spoiler window does not quite display properly. The bottom border is much too thick (how can you adjust this? I tried) and it displays the shadbottom.gif staggered on another gif. So it generally loses it's clean-cut appeal.
These 2 things fixed, would make this plugin amazing. The developer has not worked on it in some time. He is aware of the button issue, but has yet to remedy it.