I have started using Singularity now, and already I really like it.
No need for me to go into detail as Nalate has already covered it very well in her review Singularity 1.5.3 Viewer Review
I've been using Imprudence for about six months, but after using Singularity for an hour or two, I discovered that I prefer it, for several reasons...
-- It seems to be under more rapid development currently, and I expect there will be some more improvements in future - although it's already ahead of the competition in V1 viewers.
There were some understandable reasons for Imprudence's devs not being able to work on it for a while, but I must admit loyalty is not the highest priority for me when it comes to choosing a viewer. I use what I will enjoy the most.
-- It was difficult to find things in Imprudence. Their idea of a more "logical" rearrangement of menu items and dialogs never worked for me, and I'm relieved to be back with a more standard arrangement of UI items. It was easy to find everything in Singularity, and quick for me to do all the usual customization of settings needed after installing a new viewer.
-- Features galore! Siana has collected together all the neatest features from the other TPVs plus some of her own. As far as I know, no other TPV has such a complete collection of goodies - but it doesn't seem at all bloated, and performance is very good.
It will take a while to discover how it compares to other TPVs for stability. So far it's been fine.