I'm a big fan of the cinematic version of Eden Studio's Unisystem rules used in their Buffy RPG (and other games). There's no RPG system in existence that's perfect, however.
Thus far I'm pretty much sticking to the book and not using many actual house rules. Below, however, is some discussion of potential problems in the rules.
One problem I have with the cinematic Unisystem is the maneuver system. At first it seemed like a neat idea that could bring a little variety and movie-like moves to games. In practise, however, the various formulas for attack rolls and damage slow combat down. Also, restricting actions to a certain set of moves isn't necessarily a good thing.
Right now we're still sticking to the standard maneuver system, as completely abolishing it also has its problems. The maneuver system would probably work best if it was really used, if both the players and GM make best possible use of the variety it could provide. The problem is, if you're as lazy and absent minded as I am, this rarely happens.
Possible solution: abolish the maneuver system altogether. Simply make the attack roll using the appropriate combat skill and Dex. Damage according to weapon used. I'd give players the option of choosing a special effect (like disarming or knocking down) instead of causing actual damage. The same formula applies to defence rolls. Simple roll using the appropriate combat skill (or acrobatics) and Dex.
Problem #1: what about moves like staking or decapitating vamps? I'm not a 100% certain how to handle these at the moment.
Problem #2: how to encourage players to be creative in combat and not to just keep on slugging their opponents with the same move? Well, in my experience the maneuver system didn't really do much to solve this problem. As of now, I have no answer to this.
The normal multiple action rule is a little confusing and gives extra actions only to a few characters with suitable attributes. (Mostly our problem with this was that, because the rule is rather odd and applied only to one character in the group, we simply forgot to use the rule.)
One of the optional multiple action rules given should work just fine. At this point I'm using the normal -2 per extra action penalty applied to each action during the round.
The rules encourage taking turns in the order that most logically fits the action. In practice this is not as easy as it sounds, and generally we'd just end up taking turns in a set order. One drawback to this is that when facing a relatively small group of relatively weak foes they often get demolished before they even have a chance to act. More variety in turn order is called for, so I'm likely to use initiative rolls in future games.
At first I thought the Ability Scores were a great idea that made the GM's job a lot easier. After a fair amount of play, however, we're seeing potential problems with it. Fights may become too much of a routine for the GM with minimal involvement. Also, it is harder for weaker characters to get in lucky shots, and also harder for strong characters to fail occasionally. Also it takes away the GM's possibility to "fudge" die rolls when the situation calls for it. I'm thinking of using regular rules (with die rolls) for NPCs as well, but at the moment I don't have any experience of this yet.