Yes, I do all my presentations with Impress, and am often sent .ppt files from tech suppliers. All open without problem in Impress, and I can save them as either .odp or .ppt files. Likewise I can easily convert my own presentations to .ppt if I need to send them on elsewhere.
I also have a copy of OpenOffice Portable on my USB stick, so can use Impress to run my .odp files on any computer I happen to be using at the time