I've done it once at level 4. Just power trained and never applied the skill points. I just applied 700+ skill points yesterday (start level 6). I had a solid, young team of fairly fast trainers at the time, so it worked ok. Sucked missing champions league though. In a nutshell, you sacrifice one season to gain quality players for the following 2. I might do it again, but ONLY if i can't keep up with player quality otherwise. My recommendation is: don't do it unless you have a solid squad of 18-19 year old fast trainers who are considerably behind the quality curve (e.g. your team is avg quality ~37 or so if you're level 5, where the max quality you can buy is 44)