It is very easy to update and and delete data in Salesforce using this tool. some of the features that I think are particularly helpful are: uploads and updates using Excel, using constants when uploading data, updating data using business logic, identifying and performing deduplication, and insight into error records.
When uploading new records, you can specify a constant for any of the fields. So if you know all of the uploaded data is a certain record type or is owned by a particular user, you don't have to update your spreadsheet. You can just use constants.
If you have the need to replace one value with another, this tool can do. For example if we run out of stock on a product, we can upload Opportunities with a different product to replace it.
The de-dupe has many options for matching and you can build your own. You can peruse through the dupes and determine if you like the algorithm and then use it to automatically dedupe those records. There are some options to determine the master and rules for when the master has blank fields but the dupe has valid data.
Finally when records error out, Demand Tools creates a file that you can use to determine the error, correct it and reload.
Good, solid tools.
The UI and navigation aren't great. Some of the text and checkboxes are only half visible because of screen resolution. Also, you can't search for the table you need to update in Salesforce - and there are a LOT of tables. It would be much easier to use with some nav and UI updates but frankly, it would never deter me from using the tools (which is likely why they don't focus on it)