What do you like best?
The overall end-user UX is pretty good. It's simple, clean, and more or less straightforward, while also being surprisingly flexible.
What do you dislike?
Aside from being forced to do time-tracking (which isn't 10K's problem), I'd say the biggest issue is know what you're supposed to do. There's a lot of hidden functionality, buttons that don't look like buttons, links that don't look like links, etc. At the expense of making their UI clean, they've taken simple things and made them complex by hiding or masking them. There's a LOT of power under the hood, but they've hidden it all under a sometimes complex maze of menus. Once you've discovered them by playing around or had someone show you, it makes sense, but it's generally not easily discoverable. I imagine computer novices would struggle with this platform a great deal without some training or a quick tutorial by one in the know. That said, it's still pretty great once you get over the learning curve.
Recommendations to others considering the product
If you've got to force people to track their hours, you could do much worse than 10K' (I'm looking at you QuickBooks Online Edition - yuck).
What business problems are you solving with the product? What benefits have you realized?
As a consultancy, billing is everything, so it's incredibly important that we track our billable hours daily to ensure we're not giving away our labor, getting paid correctly for work done, and not over-billing. 10K' makes that pretty darn simple. I just leave the browser tab open all day and track things as they get done, and make a final pass at the end of the day.
The overseers who admin the system have a LOT of flexibility in setting up the system to do exactly what they want which is pretty cool. As an end-user I don't see that side of it, but they seem to enjoy it. They set up pre-alloted hours so we know how much we can/can't work for a given client, while also ensuring we meet our target billable hours. They add/remove/lock projects to ensure we're all billing the right hours to the right SoWs. It's all pretty well laid out feature-wise IMHO.