I’ve had good luck with workbooks less than 6 sheets. A few over that had problems which, of course, may well have been related to connectivity. I will mention, however, that I have not used any complex macros. I also keep either MS Excel or OpenOffice on the local desktop and if a file is crucial, I zip it and send it for local processing.
A general comment about hosted office suites — since most users use 5% or less of the capabilities of an office suite, I use a couple of them on community documents with others and have been very happy with the results. I also use the presentation creators for on-the-fly needs. I would not expect to use these for complex purposes, but I would keep trying (particularly if connectivity was guaranteed). I do know a few companies using the paid version exclusively (1 local app per 20 people for emergencies).