I recently picked up the Yongnuo YN560 II and will share a few thoughts.
I had tried this flash before with the normal shoe, but picked up the A mount version which does have the i-iso shoe. This also have a quick release mechanism (ie auto lock and push the button to remove) unlike the Metz units which have a screw down design (they don't contact the shoe just push the lock mechanism down)
Reading Kurt Munger's review he seems to have got a lemon copy of the flash, he says shuts down after a few shots and won't power on. I tested my unit and it does NOT show this issue. If the Yongnuo is getting hot it slows down the firing rate, I have not seen it shut down at all. The Metz flashes I've used don't shut down either they slow the firing rate if they get hot (I've yet to overheat a Metz even in scorching heat and blasting it - maybe a power pack might hit the thermal limit but normal use never a problem)
Build on the Yongnuo is good, the casing is decently made and the swivel head does not have a lock on it, but it isn't too easy to move by accident. The controls are quite logical with hard plastic buttons, you get a pc connection, port for a power pack. Also included are a case and stand (plastic threads though) You have manual, pre flash control on/off and a strobe mode. You can make power adjustments in 1/3 stops to fine tune it.
Testing the flash I can confirm that if you are using metered wireless mode with a flash off camera with TTL that the Yongnuo will not form part of the exposure doesn't matter if you set pre flash off or on with that flash. This sounds like a huge problem, but the solution is the reason I went with Metz. As I can use my flash as a "dumb manual flash" with the servo mode you can fire both the Metz and the Yongnuo off camera using the onboard flash (set to pre flash suppression) My trick is to put an empty toilet roll over the camera flash to direct light away from the subject. Both flashes are quite sensitive to light signal and have a good range in servo mode.
Currently no Sony flash supports optical/servo mode at all, this causes headaches for people wanting to use flashes like the Metz and Yongnuo in manual mode off camera. Only solution is to use wireless triggers to fire these flashes. This is the primary reason I've ignored Sony flashes for my set up. In most cases I simply need to use the flashes manual off camera, and simply trigger them via the on-board flash (you can experiment making it part of the exposure or deflecting the light so it doesn't add to it)
Obviously the Metz does support the TTL wireless and wireless HSS as well as being able to manually adjust power output in wireless. So if you're planning on getting a Yongnuo to add as a manual flash for off camera work and you don't have a flash that has a servo mode you'll find it of limited use unless you use triggers or and off shoe cable. You can't integrate a Sony flash into a manual flash wireless set up any other way bar triggers.
I read a few complaints about the Yongnuo not showing a flash ready light in the viewfinder. Simply put the flash is purely manual, it has no communication with the camera (any camera regardless of model SLT or not) bar the "fire" element on the hot shoe. The camera does not know it's there and you are basically metering with the camera only and not a flash at all. This isn't a huge problem this is how it's supposed to work. It's probably easier going into M or S mode and working from there and using the Yongnuo as you would a manual flash (A mode not so useful unless you have a satisfactory shutter speed)
For SLT models you will want to turn the setting effect off for obvious reasons, the viewfinder will go dim in low light.
Other than that I can (unlike Kurt) recommend the flash, just be aware of the intended used of it. It's not a TTL dedicated flash, it does not support HSS, but it does have a good power output, the re-cycle times are fast at about 3 seconds for full power (my metz is around 4.5) it can also fire at they say up to 8fps with reduced power output. As an inexpense additional flash for manual wireless, or a back up manual flash it's hard to complain at all. If you want a dedicated flash then you need to look elsewhere.
One advantage of the manual wireless flash set up is quite obvious. I'm not overly impressed with Sony's attempt to update wireless flash. I have to be blunt watch this video and it's far from intuitive:http://www.youtube.com/watch?v=1Gj9LU2EmYQ
Much simpler to trigger your flashes in manual mode allowing you to fine tune the exposure of each flash rather then mess around in the flash menus.
I will give some credit to Nikon, their flash system is top notch it's much easier to set up a flash system on their CLS using the camera body to set flash at the required levels. You can also mix and match flashes with TTL and manual firing power off camera. It works, and it works very well indeed.
Anyway that's my take on things I hope it's been of some use, will post a few examples shortly when I've played around with my new light tent/box!
If you want cost effective off camera manual flash, I would stay away from Sony's offerings. If they ever add an optical slave mode they might be worth a look.