What do you like best?
Super easy. You can be up and running with your own 'blog' very quickly.
Custom URLs are not too hard to do.
I suggest Tumblr to my friends who want to start a blog, but don't want to get to deep into issues like hosting, software, updates, etc.
A few tech blogs use Tumblr as their blogging solution.
What do you dislike?
Tumblr is lagging in features. You can add popups, or lead capture pages. You can't add any plugins at all. The API is pretty good, but not many people are building cool things with it.
It is very easy to do a few clicks and end up in the darker corners of the internet on Tumblr.
And it is REALLY hard to generate an audience on Tumblr. Anyone participating on Instagram can gain a few hundred followers, Tumblr you can participate the same amount and have no followers.
Recommendations to others considering the product
Great easy way to get started. If you want to add any functionality, you will have to move away from Tumblr.
For example, if you want someone to enter their email address to get a coupon, this will be very hard to do in Tumblr, while you could easily add a plugin on Wordpress.
What business problems are you solving with the product? What benefits have you realized?
If you need an easy blog, you can use Tumblr. If you get the right skin and a custom URL people won't even know it is Tumblr if not for the reblog or follow floating button that Tumblr places everywhere.