The Streetlites really aren't any worse in terms of ride quality than the comparable e200 at the time. There are clearly issues with the gearboxes but that can be sorted; another driver complaint is that the brakes can be sharp. However, I recently travelled on an example with Hulleys of Baslow and the ride and the gearbox were absolutely fine. There is also a design fault that means the emergency door mechanism rattles like anything, and drivers have complained about sharpness of brakes. However, I've travelled on them a lot (I live in a First patch) and they're ok in comparison with the e200 - not exceptional either way. It's when you compare them to an Eclipse that ride quality is highlighted but that's the same for lightweight vs heavyweight chassis. Perhaps the biggest thing is that the e200mmc came out about 3 years later and that moved standards ahead.
Also, there was much talk about enthusiasts about how Streetlites wouldn't cope etc. However, they've not been like Optare Excels being disposed of early; the first ones are now coming up to 10 years old and they're still ok.