What do you like best?
I like the ticket system and SVN integration.
The ticket system can be customized for intended workflows. It seems to work well with small to mid-sized organizations and is easy to set-up and maintain.
It's web-based, so you don't have to worry about backing up your work or the tickets. The integrated SVN repository is very nice, it's much easier to use than Hg Mercurial or Git and great for documents and Ranorex (automated testing).
What do you dislike?
All the emails. The only options are to receive updates via email or not receive updates via email. At times I will have up to 50 separate emails in my inbox. It would be nice for Unfuddle Stack to combine all updates into a single email.
"Notebooks" can be created for documentation, but it's not as good as oneNote or a wiki.
Recommendations to others considering the product
Make sure you verify that the ticketing system, notebooks, and repository works with your processes. The price is very similar to other cloud-based solutions.
What business problems are you solving with the product? What benefits have you realized?
Process creation, control, and management. It's a very simple solution for companies that don't have one. It's also a good temporary solution if more time is needed for other product reviews.