I have no personal experience with these "ultra-wide" auxillary lenses, but I'll give you an opinion anyway.
I don't like painting with broad brushes and anybody who looks down on
all auxillary lenses hasn't done their research. Nikon even makes auxillary lenses (with ED glass) for their point & shoot digitals. One of them is HIGHLY sought after by birders (to the tune of $400 for the one I sold) because it will turn a 12x Panasonic Lumix Zoom into a 20x and the pics are
superb.
Now, clearly, the lens you are looking at is nowhere near that quality. But when you go wider you get less critical (than telephoto), not more. In situations like yours, I like to find what other say about it (like Amazon reader reviews). I'd stay away from the one you are seeing on eBay and go with one like this:
http://www.amazon.com/Merkury-Innovations-CL-52WB-Definition-0-45X/product-reviews/B000HJ8P2O/
People seem pretty happy with it, and how wrong can you go for $27? If the choice is "no wide" or "auxillary lens wide", I'm pretty sure that I'd go with auxillary lens wide... particularly for that piddly price. I'm not so sure it will be great for macro (get a 52mm "reverse ring" for your kit lens to play with macro) but if the .45x is correct, you'll be getting the 35mm equiv. of a 14mm lens with the auxillary lens linked to above. Crikey, for $27 I'm thinking of trying one myself!
For more money, Bower is a pretty respected name and they make a .45x in 52mm also:
http://www.amazon.com/Bower-Advance-Super-0-45x-Silver/dp/B000K89BKM/
Only one data point for reader reviews on that one, though.
Keep in mind that most people buy these to use with their HD DV cameras, not DSLRs... but it
should work the same.
If you go with the ultra-wide auxillary lens, show us some pics you've taken with it. I'm curious to see how they look.
Don't use the .45x on the 55-200mm lens, as you already have that focal length covered with the 18-55mm kit lens.
PS... Congrats on the D40! 6 MP is plenty.