Here are my reasons:
- They are in SF so I can walk/bike/bus to their office; for whatever good that does me. Didn't do that one dude any good going to MtGox's Japan office. =/
- They are YC backed, so that means pg knows about 'em. That adds a "gold star" in my book in that they must have passed some kind of filter to get into YC; that they're not just plain scammers, unprofessional or woefully uneducated about security on the interwebz.
- I feel very confident about this video: http://techcrunch.com/2013/12/17/foundation-brian-armstrong-...
All that said, you still shouldn't leave a huge amount of coins on an address that you don't exclusively hold the private key to. 1 or 2 bitcoins, meh. Day-trading with 50 BTC? Risky, but okay I guess. Those crazy amounts I saw on that gox-horror Reddit thread, like 100+? Some people in the THOUSANDS?!!! That was a bad decision. That needs to be some place else. Coinbase.com may have the most honest people in the world, but I also believe hackers are extremely talented people and hundreds of bitcoin is adequate motivation for a certain subset of hackers to pour all their energy into breaking coinbase. I'd love for Coinbase.com to talk about all the strange hack attempts they must see against their service every... minute.
I day trade on btc-e.com with 1k USD and my rule is to never have more that 2k on there. If I reach 2K, I buy 1k's worth of bitcoin --> send to coinbase.com --> cash out. Repeat again with the 1k remaining. I could try and turn 2k into 4k, but that's a risk I'm not willing to take. I'm happy with my 1k units of profit.