First, a quick summary: the movie rental experience is pretty good, even without an Apple TV. Of course the resolution is iffy--- slightly above DVD standards by pixel size (933x470px for this particular film), but offset by the highly-compressed low-data-rate H.264. (I'd still like to know just why us non-Apple TV owners are snubbed on that particular point, but I'm betting on it being a perceived piracy risk as I postulated in BeerMediaTech 003. [no longer online —Ed. 2015-08-13]) However, the rental process itself is painless and your time remaining before the expiration is clearly laid out in the "Rented Movies" section of your iTunes library.
Transfer back and forth to portable devices (at least the iPhone I tested) is very clearly laid out in the Movies tab of your media sync preferences for the portable device. The only pain experienced is waiting for a ~1 GB file to copy over USB, even when you're transferring back to your Mac from the portable device.
The one exception to ease of use would be that you can't seem to get to your rentals from Front Row --- you have to play them from iTunes and swap into full screen mode (cmd-F) if you want a big screen experience from your Mac mini jukebox. Then, at the end of the movie, it doesn't kick you out of full screen mode either --- it just sits there on the post-credit black until you hit "esc" to kick out of it yourself, but that's a very minor complaint.
I've heard several people also complain that you need at least a 27-hour window instead of 24. This is to account for the theoretical situation of starting it at 9pm one evening, then having to stop because you're dealing with some interruption (like say kids or the need for sleep), and pausing it, only to come back at 9pm the next night and being out of luck because your rental has expired.
So here's a twist: Just to see what would happen, I started watching a rental movie again with only about half an hour left in the 24-hour rental window, and put it to full screen. Here we are now, a good 70+ minutes into the movie, and it's still playing. So, what does that mean? You have 24 hours to start watching it, and once you've started, do you get to keep watching it until it's done? Does it expire if you pause it or try to rewind? All good questions. But let's see if it lets me watch all the way to the end first.
> WAIT
Time passes...
Awesome. I was able to watch all the way to the end, credits and all, and then it stopped on the black screen as usual, no notices. Once I hit "esc" to kick out of full screen mode, I was greeted with this dialogue, warning me that if I didn't resume watching, it would be deleted:
This leads me to believe that you could perhaps pause or kick out of full screen mode, and still be able to resume the movie, though this is just a theory at this point. It wasn't until after hitting "Resume" that I guess it realized the movie was actually over and greeted me with the ol' expiration notice:
So, does that mean you can start one night at 9:00pm, and then resume it the next night so long as you start watching again by 8:59? Do you have to switch into full screen mode for this to work? ...or is it a blanket grace period?
These things I leave as an exercise for the reader (or at least until I have a chance to experiment with another movie), but it seems this might go a little way to assuaging complaints about the 24-hour window.
ADDENDUM: Yes, Nicolas Cage does look really silly with his "Vegas entertainer hair" wig on, and it was really distracting throughout the entire movie. Thanks for asking.
UPDATE: I have also now tested for a solution to the questions that remained at the end of my original post. I found that if you pause before the expiration of your original 24-hour window, you can seemingly pause indefinitely while the movie window remains open past the expiration, and then continue playing at your leisure.
However, if you press pause again after the expiration, the movie does not pause, and you are presented with a dialogue (as above) where your only choice is to resume or delete your rental. This happens whether you're in full screen or windowed playback mode in iTunes. However, you can rewind the movie after the expiration, so that could allow you to extend your viewing time even further.
ANOTHER UPDATE: Chris Breen has also discovered that there is an identical viewing time extension available if you have transferred your rental to a 3G iPod nano with TV playback. One might assume this would also apply to other iPod/iPhone models.