See the testimonials: http://apolyton.net/members/?action=test
In summary, most important advantages:
1) You're supporting the site
2) The whole site becomes faster to use
3) No ads -- pop-up, redirect, banner or otherwise
4) Bigger avatar (if you care about that)
As for myself, as a heavy PM user, I'll add that I absolutely love the extra PM storage space and the higher maximum length of PMs.
Plus Markos keeps adding more 'minor' features on a regular basis (see http://apolyton.net/forums/showthrea...hreadid=116530)