I have been using Google Spreadsheets to see how useful a tool it really is. I uploaded a small Excel sheet that I built for the purpose of testing out the GS application.
Unlike Excel in GS you have to insert extra rows and columns as and when you need them. You can perform multiple inserts by highlighting multiple rows, then right-clicking and selecting insert 5 below or above. I attempted to select 30 and insert 30 below then 60, 120, 240. When I tried to insert 240 something broke because the 240 rows did not get inserted and there was permanent Updating... message. Maybe this is just a glitch or maybe you cannot work with this many rows. I noticed that Internet Explorer's memory usage rose to ~80mb typically this is about ~20mb.
My conclusions are that currently they are only suitable for light duties, where online access and or collaberation is needed. This may with time and effort on googles part change.