I taught myself to spey cast watching you tube videos. One of the ones I watched was the RIO's modern spey casting trailer. At 1:23 in, it is all single hand spey casting. I'm sure that at some point someone may tell you that you need to overline the rod a bunch to do it, but it is NOT true. I have been doing it all spring with a 7 wt. Trevor Morgan single hand line and a 8 wt. single hand rod. (Underlined one weight) I am going to do a video shortly that shows single hand spey casting a snap-T, double spey, perry poke and single spey, and one I have been practicing, a cackhanded reverse snap-T with an around double spey. I'm doing it to prove the point you do not need to overline and there is NO waiting to feel the rod load. Your motion is what loads the rod and if you are waiting at any point to feel it load you are doing it wrong. I just need my time off to coincide with a day that has a less than 30 mph wind, which for the last week and a half has never happened.
As Ard was saying, it does make a disturbance on the water, and unless you are good enough to put a bunch of line out there you (and the fish are out that far to) may spook a few fish. It does however open up places to you that an overhead cast will not get your fly to.
I am putting two videos on here. One is the RIO trailer. When I was teaching myself to do it, I stood in front of the computer and mimmicked the motion, then took notes. Practice one cast at a time. Learn the single spey first. If you use heavier or bulky flies like I generally do, learn to perry poke/single spey. The other video is my video about Trevor Morgan spey lines. It shows a snap-T in it at 1:58. Notice that I never stop moving. There is at no point any "waiting to feel the rod load". I load the rod by making the cast. In single hand spey casting, you need to add a haul to get real distance and make up for the lack of several extra feet of rod.
One thing that helps is a longer head. My Trevor Morgan 7wt. single hand line has a 43.5' head (most lines have a 30' head) and the running line is a different color than the head. (I have been putting the color change about a foot inside the rod) This also helps but is not required. Make a mark on the line (to be placed at the tip of the rod) with a magic marker where it spey casts well. Use tape or some other temporary mark till you find that point.
If you have any questions PM me. By the way, if you have the money for the RIO DVD, it is worth every penny.