What is the maximum class memory Pi can support.

I don't think is a problem related to memory class. Instead the problem is related to internal Flash design (allowed tollerance on signal voltages and timings, for example). So the only think I can say is "Try and see what happens"

Just after the introduction of the 'pi there was a problem with class 10 cards: Linux would ask: "do you support the new fancy fast transfer mode?" and the card would say (it has to if it is class 10) YES! and then Linux would say: "OK! Lets do that then!".... forgetting that the pi hardware cannot do this....

This might be the origin of tales that say not to buy a class 10 card. As far as I know this has been fixed for 2.5 years now.....

Yeah I used some 32GB class 10s from SanDisk and a few other brands ...Yes a few don't work but the speed is only a part of the problem.

My advice is the same as many others buy one find it works ...stick with it.

Also if you are having problems from the start  I would also make sure to use the writing program under windows which has a verify or have a Block Size (BS=128k) or similar using DD under Linux so that the writing speed is kept reasonable

I used them from Day 1 no problems at least with my SAN Disk ones .....

But yes no one has really complained for ages.

I found that SanDisk flash-memory is easier to work with because it work's on multi-speed read capable.

If I have one recommendation, it's to stay away from the Samsung Plus (Quick Review: Samsung Plus 32Gb UHS-I microSDHC Card | Gough's Tech Zone) and Samsung Evo (Quick Review: Samsung Evo 32Gb (MB-MP32D) microSDHC Card | Gough's Tech Zone) cards.

For some reason, I've had many compatibility issues with them when it comes to heavy filesystem access causing I/O failures.

Other than that, I've leveraged Sony, Toshiba, Sandisk regular/Ultra/Extreme, Kingmax cards with no failures to date. I've got over 6 Raspberry Pis and most of them run Class 10/UHS-I cards just fine. The Raspberry Pi doesn't seem to be able to take advantage of higher rates than ~22.5MB/s (i.e. DDR 25Mhz transfer mode), so UHS-I cards won't be fully taken advantage of.

Another consideration is the type of I/O you're doing with the cards. As I've found through copious testing of cards with CrystalDiskMark, the cards will vary significantly - SD/SDHC/SDXC Performance Test Database | Gough's Tech Zone. The downside of this is that the faster Class 10 cards may be faster at large file sequential accesses, but be dog slow on small-file accesses. For things like updating or booting the OS, small file accesses are much more important than large file accesses. However, if you're streaming media or making a small personal NAS, then sequential speeds can be useful, although noting the Raspberry Pi won't go faster than about 22.5Mb/s because of its SD interface limits.