I have been using the Slimline 3 since it came out and before that, the Phase III dish. The Phase III was the hardest because you were hitting three satellites spread across the sky from longitude 101 to 119.
The current Slimline 3 is exactly like aiming the 18" SD dish with a small exception. It is that you need to set the tilt/skew according to specification for your location before you start. After that, all you do is find the 101, just like you would with the 18" round dish.
The three satellites, 99/101/103 are in such a tight cluster that the tilt/skew is not very critical. You can be off a good bit and still get a usable signal. If you feel that you must use a Slimline 5, then the tilt/skew becomes much more critical because of the spread of your targets. Even if you reside in a location where DTV installs the SL5 you really don't need one unless you have some local SD channels you want to see. Even those signals are, for the most part, repeated in HD from the 99 or 103 satellites making the SL5 unnecessary for most everyone.
For clarification DirecTV calls it tilt (not to be confused with "elevation")and DISH calls it skew. Skew makes more sense to me.