Using version 220.127.116.11, there are several problems with XLSX files. I've attached an XLS and XLSX made identically. The XLS file is fine. The XLSX files gives errors about unreadable content when loading into Excel 2007. It complains both about a named range and a formula.
There is also a problem with comment formatting. You can see that in the XLS it correctly autofits the comment box, whereas in the XLSX it does not.
Finally, it was my understanding that 18.104.22.168 allows more than 65K rows and 256 columns to be fully utilized in formulas. On our initial experiments, we find this does not work, and that we get spurious "circular reference" errors due to truncation of cell references to higher-numbered rows. Unfortunately, the XLSX file that exhibits this problem is rather large. How would you like us to help you debug this issue?