After my experience with Impetus at Fall In, I picked up a copy of the rules and the two "expansion" magazines. I spent most of last week reading the rules to get a better understanding of how they work. The description below is based on an email from my gaming group's mailing list. Note that I haven't had a chance to play Impetus again, but I'm going to try to set something up sometime soon.
Note: The description below assumes that you know something about DBx. I'll just comment in spots where Impetus is different.
Impetus is a unit based game. Each unit is comprised of 4 DBx stands. You could probably get away with 1 stand per unit if you adjust the scale. An average sized Impetus army seems to consist of 10-15 units, so to do it in full scale would require 40-60 stands.
Units in Impetus activate by command (think GBoH). Each side nominates a command, rolls 2d6 and adds the commander value. The highest roll activates. After the command is done with its activation, you repeat the process. That means that there is no guarantee of the order of activation, so it is possible for one side to activate all of its units before the other side.
Units activate individually, unless they are in a group (i.e., in contact with each other). This means that each unit must complete all of its actions before another unit can move. When a unit is activated, it can do things like rally, move, shoot, charge, fight in combat. That's right, combat is part of a units activation... there is not separate combat step. A unit can attempt to do multiple things, but there is usually a penalty. If it tries to move and fight, there is a penalty to its attack. If it tries to move multiple times, it must roll a Discipline check. If it fails, then it becomes Disordered. So you do have the option of pushing your troops, although there are some risks involved.
There are many ways that a unit can become Disordered. Failing a Discipline check when moving multiple times or attempting a "special" move, taking casualties, failing a charge, etc. There is a slight combat penalty for being Disordered, but there is a significant "performance" penalty, since there are a bunch of actions that a unit cannot take when Disordered. It appears that the optimum strategy is Disorder the segment of the line that you plan to attack.
Combat uses the "buckets of dice" approach. Each unit has a "VBU" rating and an "impetus" rating. If charging, you combine the values. If not, you just use the VBU rating. To give you some idea, French Knights have an 8 VBU and a 5 Impetus. Roman Legionnaires have a 6 VBU and 2 Impetus. Skirmishers have a 2 or 3 VBU and a 0 Impetus. Note that 0 Impetus units cannot charge formed units (i.e., non-skirmisher) and are instantly destroyed if contacted by them. Anyways, each side rolls the appropriate amount of dice (VBU + Impetus (if charging) + mods), scoring hits on 6s or pairs of 5s. Each side that took hits then makes a cohesion check by rolling a d6, adding the number of hits, and subtracting their VBU. If the score is 0 or lower, nothing additional happens. If the score is greater than 0, the unit takes that much damage. If a unit accumulates damage equal to or greater than its VBU, it is destroyed. If either unit suffers hits, it becomes Disordered at the end of combat. Whichever side takes the most damage (not to be confused with hits, which does make the process confusing) loses combat and is forced back. The winner can choose to pursue, and if it contacts the retreating unit, you repeat combat. Note that combat will continue until either a) there is a draw, b) the winner chooses not to pursue, or c) the winner pursues but fails to contact.
Victory is determined in a pretty standard way. IIRC a command breaks when it has suffered 1/3 losses and an army breaks when it has suffered 1/2 losses + broken commands. Note that units that route are removed from the table. I'm not sure if this applies to broken commands, but I think that it does. So once things go south, its probably going to end soon.
Overall, I think the game is pretty cool. It seems to be a bit simpler than DBx, which is both good and bad. I think the activation system causes it to be a bad multi-player game, since you have a lot of people sitting around waiting during each activation. I like the fact that combat moves units back and forth a bit, but I'm not a big fan that VBU contributes to both offense and defense. I'd have to play it a bit more to get a better feel for how things work.
IMO, its not "better" than DBx or FoG, just different. Its certainly an interesting set of rules. It definitely worth playing some more to get a better feel for how things work. I'd really like to try some games with asymmetric opponents: Romans vs Gauls, Crusaders vs Muslims, etc.