I have Ubuntu 12.04 and recently updated to 4.0.4.2 after I found that 3.5.7.2 had trouble opening larger .xlsx files. 4.0.4.2 can open xlsx files and even does autosave on them a few times but crashes every time I try to save changes to the file myself. I have no problem saving the file as .ods or .xls but would greatly appreciate any chance to fix so that I can save in .xlsx.
There have been quite a lot of improvements in OOXML handling since v4.0. I recommend installing v4.2 or v4.3 from the LO website.