Yesterday I installed an Atwood SP-400-RB Transom Saver for our 13 Sport.
I've heard several arguments pro and con on these devices. My decision was made somewhat easy because the nice wood block I'd been using to support the engine in a somewhat raised position for towing was lost during our last trip to the ramp. Our trailer is too close to the ground to allow safe towing in the engine fully down position,
I choose the Attwood for it's low cost and easy installation. Purchased at WalMart for thirty something dollars it allows for two choices of use. It comes assembled with a wide mouth "U" shaped lower end for fitting over a transom roller at the end of the trailer. This is the easiest method and no installation is necessary. Our trailer lacks a transom roller so we used the alternative method which is to attach a bracket to the rearmost trailer cross member. The attachment simply requires drilling a hole to accommodate through bolting the bracket. But, beware the Attwood supplied hardware. It's made of mild steel. I purchased stainless steel replacements from West Marine for about three dollars. The bracket for attachment to the trailer is also mild steel and I plan to fabricate an aluminum replacement this year.
The device allows for several increments of adjustment to accommodate various engine tilt settings. So far, I like it. I failed to mention that the device is designed for only use with hydraulic tilt and trim equipped engines.
I believe the device is useful because it allows for prolonged towing without any risk of hydraulic leak down causing damage to the engine lower unit by striking the ground. I also like that the engine lower unit weight is now supported by the trailer rather than the transom.