I thought I would start a thread about peoples experiences with different batteries. I'm going to post my experience and any other person's experience I think would be informative.
Me, I started off with lead and went to a set of PingPings.
1) Everyone knows about lead. Very heavy, likely to be dead and worthless when you come across a used one, short lived and not so cheap if you are more then a casual bike user. The only use I see (for us serious bike users) is if you are going to sell the bike. Put some cheap lead batteries in the bike to sell it, keep the better batteries for another bike.
2) I installed some Ping's in my bike five months ago. Ping is a excellent shop to deal with, the batteries are a better deal then lead do to the longer life, half the weight of lead, more power then lead. The only downside I see is the low C (low discharge rate). This required me to buy more Ah's then I needed, but then again it's nice to have the extra reserve and not drawing the batteries way down should make them last longer.