Sorry for such a late reply. They do make a big difference I think. Anything besides straight on flash will work, and they provide surprisingly nice light. Here's my short run down on them.
Lightsphere: This is odd kind of beast. Nice light, but flat. Until you get used to it, I find that results can be unpredictable as well, since it throws light in every direction. This can actually be a big benefit though if you're near walls, and you will see the difference. The introverted dome that it comes with is very poorly designed though. It's purpose is to push light back down and around, but I find that it pushes too much light front (in extreme cases creating more shadows) and can be unflattering up close. I wish the dome was adjustable. Shooting with the dome off can wield nice results, but something looks off at times. If you have the dome on and aren't too close, you'll be fine. You can even use the dome straight forward (sometimes I like that better), but without any walls present, be prepared to bump up the FEC. Remember that even straight on, it's still sending out light left, right, down, and up, even though it's a very small amount. The fit is very tight, but I don't think it will ever fall of the 580ex II (model P4/C4 is made for it). You have to stretch it a bit to even get it on, but it's easy, and I've been able to hold my XTi with 28-135 for about a minute just by the diffuser alone. It doesn't make the flash fall when pointed up, but can it can move the head when pointed left or right. Taking the introverted dome off does seem to pull it up just a couple mm though if you do it one handed. I have both versions (clear and cloudy) and I would only get the cloud if I were you. I find a minimal difference between the two, and think they're overpriced at it is. Do not purchase it from Gary Fong's official site if you can, or resort to other online merchants, as I find Mr. Fong's shipping too expensive.
Flip-It: I haven't used this a lot, so I don't have much to say, but I find it's more predictable than the Lightsphere, since it's a 2 light source, and it also has less light loss, so it should use less power. Close up it's better as well, much softer light. The ability to tilt the bounce card is also very nice, but even the slightest tilt back (for softest light) will require extra power. The Flip-It can be as soft as the LS I think, but you have to keep adding power, more than I expected. I actually have the DFD-Pro Jumbo, but find that the diffuser doesn't do that much, so you might be able to skip that. The larger size is supposed to help with softening it, but I don't have the square one to compare it to.
Final comparison: DISTANCE- The Flip-It works good close or far away, since it doesn't have plastic to go through. The LS is just as good, but can be too strong straight on, and will require more power at longer lengths (to be expected). / QUALITY- The LS light is very wrap around light and can be seamless. The Flip-It produces similar results, but it's soft setting is very tricky, as it quickly requires more power. It will also show shadows quicker due to design, but I haven't had any real life problems. Up close, it is much more flattering than the Lightsphere because of no dome. / ON FLASH- Lightsphere can pull at flash when pointing left/right, but isn't too bad. Flip-It is way lighter, but I don't think it's velcro strap is as tight as the LS's opening.
I would buy the Flip-It first. It seems much more versatile and predictable, plus the light will travel further since it has no plastic to penetrate. It really beats the LS all around since it's so simple - no plastic, easy mount, very lightweight and small. The only problem is that if you're not using the tilted back position, even for the price it it's expensive since there are lots of similar DIY techniques. The LS, although it works good, seems like a more specialized product. It still has it's purpose, but might not be right for all situations or might require more use to get the hang of. The Omnibounce would better to compare to the Lightsphere than the Flip-It.
I don't have a Flip-It example (haven't had much use with it yet), but here's one of the LS cloud recently (ignore the weird 1/40 f5.6 ISO 400 - accidently shot on Manual Mode).
full link (not larger):
http://www.flickr.com/photos/16976576@N00/1836295902/ 
