I thoroughly recommend IS (stabilized) Canon binoculars and I have a few to prove it:
10x42L - original purchase, good general purpose, originally for astronomy but wanted more aperture/mag
8x25 - second purchase, great for theater and stage, and lightweight walkabout
15x50 - third purchase, perfect for astronomy and wildlife, more magnification gets tough to handhold
I have tried a friend's 18x50 and they are fine -- but the key point about IS is to allow handheld steady use and there is always going to be more "jiggling" at 18x rather than 15x, even with IS, so I am extremely happy with my 15x50 for my purposes.
I don't see an optical quality difference in practice between the L 10x42 and non-L 15x50. All have crappy lens caps of various designs (or none at all). The L has a more padded case. The 8x25 takes less common CR123A batteries, whereas the other two take AAs. The 8x25 annoyingly needs you to hold the IS button down -- the other two have a time delay to hold it down for you, then releases to save the batteries. Battery life seems good on all (never been a problem for my use).
In summary, they are an expensive indulgence but I would never go back to non-IS. If you can afford them, I recommend them to all -- especially astronomers and wildlife enthusiasts.
PS. Look for deals on used. My 15x50s were well under 1k on eBay this year. See also BH/Adorama used. http://www.canonpricewatch.com/