[Update February 8, 2013. I haven't updated this page in a couple years so I'm not sure when Impress improved, but LibreOffice just released version 4.0 so I figured I'd give that a try. All of the slides in this test are now rendered correctly, as shown below. I'm still a bit annoyed at the apparent lack of anti-aliasing, and as a result they still look better under PowerPoint. Also note that these slides aren't the complete story; see slide 13 as displayed on the original page; apparently PPTX presentations that use the "new" fancy shapes introduced in 2007 are still hopeless. But at least it's now possible to make a PPTX presentation that Impress will render correctly (without editing the master slides), so I guess that's good news.]

These tests are much simpler than the ones from my actual presentation. (In some respects, this isn't really true, but it's good to be sure.) What I did was as follows. I opened PowerPoint 2007 and let it create a default document. (For those who only use OpenOffice, this consists of a presentation with one title slide. The title slide has a title and a subtitle text box, both of them "ephemeral" in the sense that if you don't modify the contents, they are invisible during the actual presentation.) I then modified it along two axes:

• I either typed something in for the two title fields that are present ("titles") or selected both title fields and deleted them ("blank"). (For the curious, leaving them present but ignoring them doesn't help matters, and in fact hurts: the ephemeral fields don't load very well in Impress despite it supporting that feature in its own presentations.)
• I did one of the following:
  • Left the background alone ("white"),
  • Right clicked the slide, choose "format background", and set the background to solid red ("red"; yes, this is really ugly), or
  • Used the "format background" in the "design" ribbon to set the background to a solid tan.

This gave six presentations, which are shown in the table below. For the most part, the PowerPoint screenshots are taken from the editing window and the Impress screenshots are taken from presentation mode, but in both cases it wouldn't make a difference, all the slides look the same both ways in both programs since there are none of those ephemeral fields. (If you're curious this was just for expediency; I took the PPT screenshots first and it was way faster to take them in-window. When it came time for OpenOffice, I wanted to show that the "click to edit the master subtitle" field doesn't disappear during presentations, so I did those from presentation mode, but I didn't want to go back to PPT to redo those.)

Here are links to the actual presentations:

For screenshots we'll start out with a pair of representative comparisons so I can show slightly larger thumbnails: both of the ones with white backgrounds. (The background-styles variation shows a new problem -- Impress doesn't honor that setting -- but that's harder than I initially thought, as it gets the background right when set a different way. Other than that, all the other comparisons are really the same.)

	PowerPoint 2007	LibreOffice 4.0	Impress 3.2
titles
blank		[LibreOffice 4.0 renders this slide correctly]

And now the full results (note that LibreOffice 4.0 renders all blank slides correctly, so I didn't bother putting up screenshots):

	titles			blank
	PowerPoint 2007	Impress 3.2	LibreOffice 4.0	PowerPoint 2007	Impress 3.2
white
red
background-styles

So what have we learned? (Note: most of the below was fixed between OpenOffice 3.2 and LibreOffice 4.0)

• You can have a slide which, in PPT, appears to be completely blank (as in you can try to lasso the entire slide and select no object) and Impress may still insert a text box. (In this case, that text box is present on the master slide. However, it does not show up in PowerPoint either in edit or presentation mode.)
• The above continues to hold if you leave the ephemeral "click to add title" and "click to add subtitle" text fields in place and fill them out. (Knowing that it's coming from the master slide means this makes some amount of sense.) The result is text overlayed with text.
• The above problems do seem to be confined to the titles slides: other slide layouts don't have this problem.
• The text formatting is wrong on my subtitle. It should be grey and centered; instead, it's black and left-justified.
• If you right click on a slide and choose "format background" and set it to a solid color, it works (that's the point of the ugly red background), but if you use the "background styles" dropdown in the "design" ribbon, then it doesn't; even if it looks like it's just setting it to a solid color. (This is the one place where I think that you're right about my original comparisons not being entirely fair.)
• While we're at it, look at how crappy the not-anti-aliased text looks. (Is there a way to get it to? Perhaps. I'm using a fresh, default installation. But if there is, why the heck isn't it on by default?) This seems to be a problem with Impress in general (and they seem to be working on it), not the import filter.