Presto may have been a quality engine, but so many sites didn't render properly on it (or simply refused, necessitating user-agent hacking) that it's hard for me to miss it.
Modern web is so broken that it doesn't render "properly" in any browser. There is no "proper" rendering. Not anymore.
Since I still use Fx 3.6 as the main workhorse browser, I use Fx Alpha (aka the rolling release shit #2) and Chrome (another rolling releases crap #1) for the occasional pages which do not render properly.
Funny thing. The sites which are most certainly broken on Fx 3.6 are often most certainly broken in the other browsers too.
Even some high-profile web sites are quite broken in many places.
The most infamous example is the imgur which causes every browser (I tried all: IE, Chrome, Fx, Opera and Gecko/WebKit clones) to go quickly above 1GB RAM consumption, eventually either crashing (typical for Fx) or going into heavy unsufferable swapping (typical for Chrome).