I use EPPLUS at the client level, we dont support servers yet. Note that it is 100% complete in regards to everything it supports (such as advanced charting styles), but it is VERY fast and should support 99% of what you need. I prefer it wildly over Interop, you dont even need to have Excel installed. I used to generate reports with 500,000 row items that took Interop over 20 minutes to create. The same report in EPPLUS took less than 5.

The main problem with Interop is it is so slow. EPPLUS is only one of many solutions though and its all a matter of taste. The thing I liked most about EPPLUS is that the coding was almost the same as writing macros, so there isnt much of a learning curve to it.