I've upgraded my E-fun A/Erato 1500W/48V/36Ah SLA machine to Lithium (48V/30Ah). The pack is built from headway 38120S 10Ah cells in 16S3P format. Physically each K-long SLA battery is replaced by one 4S3P pack of lithiums. The bike got 30Kg (66lbs) lighter and is now nippier away from the lights (no need for 1st gear any more) and there is no performance drop as the batteries discharge there way there was with SLAs. Range appears to be significantly increased. Overall I'm very pleased, and would say that lithium is much better and worth the extra cost, assuming the longevity is in practice somwhere near what is marketed (2000 cycles).
The actual replacement onvolved taking off quite a lot of rear trim as one pack is quite enclosed. I have created a pictorial to show how this is done because it's not at all obvious. http://wookware.org/pics/moped/lithbatts/
More details of the upgrade are here: http://electricmotoring.forumup.co.uk/viewtopic.php?t=166&postdays=0&postorder=asc&start=15&mforum=electricmotoring
I have also added a cycle analyst which is reporting 33Wh/km (52Wh/mile).
One unexpected benefit is much more efficient charging. LiFePO4 seems to have much better charge acceptance (nearly 90%) than SLA so I am using about 800Wh to recharge rather than the >1kWh I was using to charge the SLAs.