Hi to all:
I have both the -30 and -42 version of the Canon IS line, and enjoy them immensely. As has been outlined, the 30mm for it's compact size and handling, and the 42mm for it's better optical quality and stabilization. My version of the 30 takes a few seconds, maybe 4 or so, to come back into clear focus once the mechanism is engaged. I do not see this problem w/ the 42mm glass. There is perhaps just a shade more shimmer, or oscillation of the image w/ the 30 than w/ the 42. I don't think I would have noticed the waver if the bins had not had IS- my view is that hand shade would've drowned that part out. Basically for me, I had to decide which is more bothersome, hand tremor or the perculiar small scale judder the Canons show. Again just for me, I have seen several advantages of an IS system, however imperfect and however they might suffer optically in comparison to the traditional alpha bin, over a non-stabilized glass in a number of areas. They work great when glassing ducks, geese or shorebirds from a car w/ the engine running and are especially handy for birds at a distance, where the image scale is small enough that my hand tremor obliterates any detail. I have taken to using them exclusively in the tropics, for example, where sorting thru swirling mixed feeding flocks at a distance, in the tops of trees tossed by the wind is a frustration w/ non-IS models. On a recent trip to CR, I managed to put Blackburnian Warbler on the list, by conclusively spotting the white "tram lines" on the female's back. Don't think I could've managed that w/out a rock steady view. It's a pity, really, as I now have a drawer-full of bins laying dormant, including both 8X and 10X SE's and a couple of Zeiss 7-bys, FL's and NightOwls. Of course, YMMV as they say, but since I have become acclimated to IS even 7 power is just a little wobbly, unless I am well supported (like sitting in a chair, or propped against a tree). Guess I have lost my eye-brain coordination for regular bins.
Yes, the 42's are heavy, and shaped like a mini-fridge tipped upwards, but the glass has a very sweet unity of focus across the FOV, ample AFOV (larger than the 30) and clearly less CA. Personally, I have compared this glass w/ the 10X SE, and don't see a lot of difference, but maybe these old eyes are not the best! Certainly the L glass is better optically than it's little brother.
So, for the OP, if you are really sold on IS bins, then yes, I would say spring for the 42's. Consider not only the optical and mechanical quality, but as others have mentioned there are other elements to IS that (for me) kick the zen factor of birding up a few notches.
regards,
UTC