I like the Opera browser on my N800. I tried the MicroB engine on the N800 browser and also the Minimo browser and found both to be inferior performance or usability wise.
For example, the MicroB engine takes longer to display big web pages like the Wall Street Journal (http://www.wsj.com) while Opera is super fast. Also, the MicroB engine doesn't allow touch scrolling on wsj.com while Opera allows decent touch scrolling.

In case of Minimo, the touch scrolling/panning is useless because on touching the screen, it reverts back to non-touch mode.

I will be looking forward to improvements in these limitations with the new N810 OS which will probably also be available for N800 users with a newer browser. AJAX is less important to me compared to the performance Opera gives me on sites like wsj.com.

Keeping my fingers crossed...