I use a Touch Diamond (non-pro), and have got several browsers on it - horses for courses as it were.
The bundled Pocket IE is fast and quite frugal on resources. It's good for light browsing, downloading a document etc.
Opera Mobile 9.5 is very good for everything else, except it can consume quite a bit of RAM on occasion. It doesn't feel as fast as Safari on iphone but that's due to the way it renders the pages, which it does quite cleanly. It can also do "save link as.." (even having a seperate downloads tab) as well as being very clever in reflowing pages when needed.
Iris Browser looks good, quite functional and speedy. I don't use it as much, but is quite good.
I only use Opera Mini when a website is too laggy/problematic. The proxy thing does wonders. But because it is not a native app it's a small hassle to go through the JBlend.
I had tried the MiniMo. It was a hog of a thing, lasted all of 15 minutes on my device, five of which was probably waiting for it to boot up.
Firefox 3 is already quite a resource hog on the desktop, fortunately my deskside can take it. Its strength is also its weakness, namely the extensions. It needs quite a few to reach what Opera already has well integrated. I've switched to Opera on my Acer Aspire One, it's a lot zippier this way.
If Fennec to gain any foothold in the pocketable stage, it needs a core much leaner than its desktop counterpart.