Thoughts on using .co domain for a startup?

There was someone on HN a few days ago who owned a .co website and said that he received a lot of emails from people telling him that his logo was missing an 'm' after '.co'

That's why I think a lot of people would append an 'm' to links to your website, assuming that you made a mistake when typing the link.

I think it's better to use getXXX.com than XXX.co

Don't use it. It looks dodgy, it's easily mistaken and has no recognition at all. No matter how much GoDaddy pushes it, this is yet another attempt from a privately held domain registry + ICANN to monetize on a new top level domain, as they did with .pro (100 usd a pop), .us, .name, .biz and many others. Go for whateverXXX.com and don't even bother for the other extensions.

I moved my main domain from theminimgroup.com to minim.co when .co domains came out to get a shorter, better looking domain. I've had maybe 1 or 2 people ask me if I meant to say .com on something but it seems like the overwhelming majority don't have any problem with it. Then again, I wouldn't know how many emails I receive at minim.com.

get all three, and redirect them to getXXX.com

Whilst .co is rising in popularity (and Godaddy are starting to throw their weight behind it), I'd still avoid using it mainly because consumers are so used to the .com extension.

However, if you really want the .co extension then you could purchase all 3 suggestions you mentioned above - XXX.co, getXXX.com and XXXapp.com and then redirect the other 2 domains to one of them (most likely being getXXX.com)