Recently I went to Korea for holiday. I don't want to carry my 580EXII due to the weight so I was looking for a cheap, good and small flash.
The flash that I got was the 270EXII. It costs about HK$980 ( US$125 )
You did not mention you are planning to use the flash in a studio or outdoor or travel, so I am going to share my experience using it as a travel flash in a simple walk-around setup.
Canon EOS 60D, EF-S 15-85mm, 270EXII
Some criticism that I read online about the 270EXII:
28mm or 50mm only, no 24mm (notice my lens wide end is 24mm equiv.)
under powered
pricy compared to 320EX, which is also pricy, and 430EXII (best bet of money)
I tested it in shop, firstly the underpowered statement. It's twice as powerful as the internal flash, which is GN13 on a 60D. As far as I know, most DSLR have internal flash that has a GN rating around that range, which is considerably more powerful than the tiny flash you find on point-and-shoot. When I was in Korea, in bright daylight, I can use it as fill-flash without problem at f/8.0 and ISO 100. If I need extra reach, bumping the ISO to 400 is usually enough to fill in some light on subjects that is more than 20 meters away.
As far as travel/holiday photography goes, 270EXII is powerful enough as long as you are not doing something like trying to overpower the sun.
As a fill, the 28mm didn't really bother me much. Sure a 14mm wide panel diffuser would be helpful, but I don't miss it a whole lot.
At $125 it is pricy compared to many other non-canon flashes. But you get a few things in return:
Good build quality, dropped my camera with the 270EXII attatched at knee height TWICE during the trip, not even a scratch.
Near silent recycling (on website it says its silent, but its not, you can hear it if you are in a quiet room)
Relatively fast recycling, MUCH faster than the internal flash. If I need to shoot in burst, I bump the ISO to 400 or 800 and the flash can handle it quite well. It is still nowhere near the 580EXII's 5+fps at ISO100 and f/8.0 but when you know the 270EXII's limit and adjust for it, you can still get good results without too much compromise.
Wireless capability. This is quite handy. Like you said you can use this when you need to bounce off the wall BEHIND you because the 270EXII doesn't swivel. Usually I bounce off the ceiling and I occasionally add my palm as a giant catchlight to reflect some warm fill-light on my subject. However if bouncing off a wall BEHIND you is an absolute MUST, the 270EXII will force you to use the wireless function all the time. If your body doesn't have the wireless flash function, and you need to bounce behind you, maybe the 270EXII is a poor choice.
I use my flash as a travel fill-flash. It is light weight, this is something that everybody knows, but in practice this actually translate into something extremely useful. I attach my flash on my 60D 90% of the time, without needing to take it off at all, since its so light. At the flick of a finger I can switch the flash on/off depending on the scene. When I do take it off I put it in my pocket, NOT possible if it was a 580EXII! This means less bag-opening, zipper pulling, mounting and locking etc.
It is just right there, ready for action, ALL THE TIME.
It's colour temperature is also accurate as you would expect from a 580EXII. It works well with RAW + Auto White Balance. It will switch to Kelvin when you turn it on, and switches back to Auto, or whatever you have set the WB to be, as soon as you switch it off.
It takes two AA batteries, good for weight but you better carry extra batteries. Starting from 2 fully charged AA battries, I had to change battery just before I filled up my 16GB SD card (18MP RAW) to retain good recycling time.
Overall, you get the usual standards of a Canon flash, with wireless, and is considerably more versatile than the internal flash. It is powerful enough for some fill light and indoor bounce, and you can also up the ISO to compensate for power. If you are willing to accept ISO400/800, the 270EXII is small, cheap and performs well enough that shall fit your need.
Hope this is helpful to you and give you a practical view of the flash in operation.
Edit: sample photos
Indoor, bounce off white ceiling, against bright sun-lit distant background (f/8.0 ISO 160)
Outdoor under direct sunshine as fill, bounced off white wall close to myself (f/8.0 ISO200)
Indoor bounced off white ceiling (f/7.1 ISO 400)
Outdoor, bounced off the white tent to create a giant and soft light from above. Without the flash this photo will appear to be more side-lit and the background will be even more blown-out. f/8.0 ISO400
Act as a fill to give a little bit more light into the shades. I messed up the colour temperature because I was using the flash with no gel filter, and it was very early in the morning (warm sunlight, probably under 5000k) so the fill-light gets a bluish tint.
However this shot was taken at the 24mm end, and I have trimmed it down. The building that you see was actually 30 meters away from me. Notice there is some details in the shadow. Without the flash, they will simply be to dark to show any details. Notice the yellow square on the right is brighter than the one of the left due to the difference in distance. It is not a good shot but it shows how far it will reach under daylight. f/9.0 ISO 400.
