Yesterday a user on Stack Exchange asked me why I prefer manual over TTL when working with speedlites. I use them mostly off-camera and I've developed a kind of a workflow where I do not walk back and forth between the light and the camera more than a few times.
So my reasons for manual over TTL are:
What if any are the advantages of TTL over manual in off-camera use?
If you're setting up the lights, and they're a fixed distance away from your subject, then use manual. Other than if you fire your flashes with insufficient power, your exposure will be consistent from frame to frame. That's the boring example. Nothing is moving. TTL doesn't gain anything over manual.
If the distance isn't fixed, then it's still possible to shoot manual and alter your f-stop to control the exposure. This works relatively easily with a single, fixed light and a moving subject. I've done this successfully with a studio strobe and a toddler.
If the distance isn't fixed, and you have multiple flashes, and the distance between all of them and your subject may change rapidly, then TTL will help preserve what little sanity you may have left. The dance floor at a reception to a wedding can be an example of this. Subjects are varying distances from the flashes. The flashes may be moved quickly to set up for a different type of lighting. I've also done this successfully with multiple flashes and multiple toddlers.
With TTL you can get a decent picture in places and situations where it is hard or impossible to setup your lights. For example, you can shoot a very decent quick & dirty portrait by holding your flash on your left hand as far left as possible and shooting with your right hand. In a situation like this it would be difficult to mess with the manual controls on your flash or your camera, since you have no hands left!
Even though I, like you, prefer to shoot my flashes in manual mode, the argument that you have less control with TTL is debatable. You are really transferring control from the power setting to the flash exposure compensation setting, so in my opinion it ends up being about the same in terms of control.
It's a lot like the difference between manual mode and the semi automatic modes on the camera (M vs. A/Av or S/Tv).
In TTL you can get an ok exposure every time on the first shot, you can also theoretically set your exposure compensation in advance and shoot without adjusting anything (because you really care about the ratio between lights not absolute power) - so TTL is faster (at least if you know what you want and don't experiment while shooting) and safer (for example, if you can take just one shot with no testing TTL may be a better choice).
As Miguel said, you don't lose or gain any control because you only exchange the flash power setting for the flash exposure compensation setting.
I personally use only manual flash off-camera because TTL remote triggers are too expensive for me, but if I want to be sure I get a good exposure on the first shot I'll use a TTL flash on camera (and use reflectors and flags to control the direction of the light)
Remember that TTL will make fill flash calculations trivial as you can use the in-camera spotmeter to read the background light, then shoot your subject. Dial the EV compensastion up or down as you wish to either have more or less fill and you're set. The camera will make the proper exposure for the subject as you've told it that the background is the target illumination level.
Put a 1/4 CTO gel on the flash and you've got a nice warm glow on people's faces, even in shade.
Makes getting good pictures of people enjoying summer activities a breeze.
If you read Joe McNally's books, DVDs, seminars, etc. you will see that he swears by TTL.
If you read David Hobby's books, DVDs, seminars, etc. you will see that he swears by manual.
They are different. They are not always better. Each has times when its great, and times when the other is better.
I watched Joe McNally do a seminar in Washington DC, and his Nikon CLS flat out failed him. A manual setup would have worked. I'll never be as good as Mr McNally is, but when something fails for him in front of 500+ folks, then it clearly can't be fool proof.
If you will be using your camera in a very tight event such as wedding, cosplays and birthday parties a TTL flash is the one for you but if your doing some strobist work I suggest to get manual strobes. As David Hobby says it's much better to have full control on your strobe and it is much cheaper, I mean way lot cheaper.
Currently I have 3 Yongnuo 560 and I don't feel needing a TTL flash as of now.. If I will be attending an event I said in my statement above I will be needing a TTL strobe for ease of shooting.
[2014 update] There are two advantages in having TTL-capable speedlights for off-camera use, if you also use TTL-communicating radio triggers: you can have true high-speed sync off-camera, and remote power control over your lights.
TTL capability doesn't just mean you have TTL as an auto power-setting mode. It means you have the full hotshoe/flash communication protocol. With triggers such as the PocketWizard Mini & Flex units, RadioPopper PXs, or cheaper TTL units from Cactus, Phottix, Pixel, or Yongnuo, you have the convenience of being able to adjust the power from the camera. You may also be able to set groups differently or turn groups on and off, or set zoom, sync, and even custom functions from the camera menu as well. But for this full function, you must have TTL-capable flashes.
The push is now on by 3rd party manufacturers to add remote power and group control to manual lights as well, but it's rare and typically ties you into a specific flash & trigger combination (e.g., Godox V850 or Wistro barebulb flashes and the FT-16/FT-16s combo; or YN560iii and YN-560-TX). And HSS cannot become part of that equation, given that manual flashes by their very nature, are not capable of it.
