Good:

1. Travelling more
2. Paying less
3. Hotel upgrades, 100% success rate so far
4. I shop around a lot more to find better deals
5. Met some interesting people, both online and in the real world

Bad:

1. I have a stack of credit cards about 3 inches high
2. I have an inch of gift cards too
3. Buying something online now takes a lot longer than before
4. Spending more money overall in order to collect miles (although I manage to convince myself it's stuff I would have bought anyway)
5. Spoiled by all the upgrades etc
6. I find myself preaching the benefits of loyalty programs to friends and family who couldn't care less and are happy to book Y travel and standard rooms via $random agents and websites.
7. I have stopped supporting my local moms and pops establishments in favour of corporate behemoths who accept Amex, often requiring me to go out of my way to do so