If I'm honest, very little. The user interface is fresh and quite nice to use and I like the bolt-on product Payroll which you can add to Book-Keeping.
The email support team (which I have had lots of experience of) are quite responsive, but don't necessarily achieve resolution.
Mainly, I bought on price and the price is less than the likes of Xero. However there is a reason for this!
I really bought in to the fact that I could link my business bank account (Barclays) to the software to automate the flow of transactions. Before buying, the sales team were very active and constantly said this was not a problem. In reality, it does not work and it took several calls to both Kashflow and Barclays to establish this. They have now woken up to this reality and there is a statement that confirms this. However, the team that were very friendly and help in the sales process don't even reply to emails once you've bought!
Also, the settings menu is not very user friendly in book-keeping unless you have accounting experience. I now understand why Kashflow was recommended by an accountant!
Also, there knowledge base is terrible. Even searching for a term such as P11D returns no information on the Payroll product. and searches often take you through to their IRIS support pages rather than Kashflow product.
Finally, on the Payroll product, I experienced an incident with the software which I felt was a problem with the software. There is no audit trail for the support team to refer to, therefore, they could not identify whether the issue was user created of a bug in the software.
Also, there software issued P60's with errors and had to re-issue the P60's a month later, more admin for me!
All this and I'm just a basic two-man band with simple requirements!