Joe,
I have the Canon 35mm f2.8 - not a Serenar - mine is the "II" according to the Canon Museum. The only real difference that I can see from their specs is that the II has a 40mm filter size. From observation of the Serenar (the "I" according to Canon), the Serenar also had a completely chrome body - mine has the black like the 50mm f1.4. The Serenar was circa 1952, mine is from 1957.
I've never been entirely happy with mine. Nothing 'wrong' with it, just doesn't seem especially sharp - and not much for contrast, either. I mean, it is not bad, but it didn't even compare to my chrome Canon 50mm f1.8, let alone the black Canon 1.4.
I have an A. Schacht Travegon 35mm f3.5 and I use that instead. Also not as sharp as I would like - so I tend to skip 35mm and go to 50mm!
I could use a nice sharp 35 or 28 in LTM, but haven't seen one a price I like yet. My belief is that you didn't miss much on the Serenar - and that's coming from a Canon-loving guy. It's probably best for guys rounding out a collection. Just my 2 cents.
Best Regards,
Bill Mattocks