DW,
66 blocks are made cat5 and cat5e (data) compliant.
They might be old, but they're tried and true. And if you use a split block, they're just as condensed (if not more condensed) than a 110 block. By split, I mean that you can fit 100 pairs on them (those types of blocks have tails coming out the back and are more commonly used on entrance protectors).
Standards compliant.
Hmmm, yes I do see more 110 blocks going in -- but just because they're standard doesn't mean that you have to agree with them. I certainly find it faster, easier, and more efficient to work on 66 blocks.
See a post a while back about BIX blocks, same applies from my standpoint.
I hate the adapters you have to use on a 110 block in order to use any device with ABN clips. I also despise the difficulty in finding a tone on a 110 block, whereas with a 66 it's much faster.
66 blocks are cheaper. 66 blocks are easy to label, and yes they make 66 blocks that have desi strips too.
Two schools of thought. I work on 110 blocks every day, and I hate them. 66 all the way.