Someone wanted specifics, so here is a specific, reproduceable case.

I loaded 6 CSV files each with 7,200 records of time and sound level data
(I'm an acoustical engineer) into individual sheets in one workbook. I've
done this for years with 2003.

Drawing a line chart with three of these sets plotted against a common time
scale, dragging the graph around, etc. is significantly slower in 2007 than
2003.

I copied the graph from one sheet to another to edit it to refer to the
other three sets of data. When I click on one of the graphs to select the
series, 2007 sits, and sits, and sits...minutes go by. Task Manager reports
"not responding."

I saved the sheet as an XLS file and loaded into Excel 2003. Near
instantanioous response. Same moodel machines, both running XP. Also tried
on a MUCH faster dual core machine, with no obvious improvement; app still
sits and sits.

If someone from MS wants to look at the files I'll be glad to send them.

At this point 2007 is nearly unusable for me.