Not really felt the need for S-ones, although a good set of straight ring (and ratcheting ones too), and a few swan-neck ones in select sizes (13, 15 and 19mm) are a must-have in my tool box now. I can't stress how good the ratchet ones are though, so useful!
For DIY I would not bother. As a pro mechanic I have a set but use them very infrequently. Most of the time I use mine is doing a wheel alignment where some of the bolts are hard to get at with a straight spanner.
A c shaped 10mm did make removing the blower housing on my P38 much quicker tonight. An s would have done just as well. It is the first time I've ever really used them however!
I find a C and an S spanner invaluable for getting lower exhaust manifold bolts out (and back in) on the V6 in my TVR but can't think of anywhere else I've really found them a "must have"
