Just trying it out, and it looks awesome - just for encounter design alone, its far superior to monsterbuilder (even the old one, given how easy it is to print of sheets with the monsters you need - well, in theory - see below ).

One thing, though... is it possible to program it to have more than one dice roller open? For example, it would be nice to be able to have a d20+ whatever open for one's attack, and a d8 + whatever open for one's damage.
Oh, and I guess another thing... is there some kind of guide as to what to type to get which icon? For example, if you type an "m" before a power, you get a melee icon... I was trying to get a close blast power, and couldn't figure out how to get it to work. Sure, I could pull up a monster with such a power and shift click it to find out... but a list of such things under the toolbox would be handy. Different icons for close burst and close blast would also be nice...
Oh, and one final thing, you might make "skill power" one of the autoprompts for "Class and level" when creating powers.
Oh, and I'm not sure why, but the numbers I got when changing the levels of monsters don't always jive from what I get when doing it in monster builder - for example, I bumped an Earthquake Dragon from 9 to 10, and got 212 HP (vs. 224 in MB), Reflex 22 (vs. 23) and Will 20 (vs. 22). Not sure why or whether you or MB is "more correct". It also doesn't remember if you've already clicked on the "high defence" button for elites and solos.
Likewise taking an Earth Elemental from 11 to 10 gave me 106 HP (vs. 11), AC 26 (vs. 25) and F/R/W of 23/22/22 vs. 24/19/21.... so something screwy is happening systematically somewhere.
I think I figured it out - it seems to be recalculating the monsters stats, which in turn affect their defences. Be nice if in the advanced options where was a "do not change" option in addition to the "pick high stat" one... indeed for all the options in the "advanced option" bar - I want to change the level of the monster without needing to double check I'm not changing its role, and stats, which seem to default to soldier and str, respectively.
Actually, leveling also doesn't seem to be affecting the plus to attack... not sure what you can do abotu that, though... though oddly the damage seems fine.
And... the rendering to print wouldn't work properly - I'm using IE, and all I get is those little boxes with red "x"s where the cards should be.
Oh, and one final thing - just discovered zoom out, which is awesome... but is/would it be possible to name the various screens in that view and/or have the background of the main screen match that in the zoom out?