The Tessar formula is stretched by using it for a wide angle lens, and it shows in the Elmar 35. It's beautifully made, of course, but is indeed quite soft in the corners wide open, with a moderate dose of light falloff. Fine lens for travel shots, stopped down a few shots, to be enlarged to 4x6, but not for critical enlargements.
The price/performance ratio for the Elmar is really bad, because it's a "gotta catch em all" collectible. A 35/3.5 Summaron, especially the earlier A36 filter version, is almost certainly cheaper than the Elmar, and undoubtedly better. The late E39 filter Summaron is more pricey, and the (genuine) 35/2.8 LTM Summaron is crazy expensive ($ thousands).
I got an Elmar instead of a Summaron, but it was because I wanted a period peice set of lenses to go with my IIIa. But I was very patient to get one for $140 on eBay (from Latvia), most of them were selling well north of $200.