I've used a modified version of the Unearthed Arcana rules for a couple months in a D&D game.
My modifications:
1. Characters add their level to their number of wound points
2. Instead of damage going directly to wounds on a critical hit, crits deal damage normally to vitality points and then deal only die based damage to wound points.
3. Characters who take wounds have to make a save or be stunned for 1 round not 1d4 rounds.
4. Characters are only fatigued when they are completely out of vitality points, not as soon as they take wound points.
5. NPC classes get vitality points.
6. Some untested rules that I never ended up using because the campaign is only at 6th level so far: Keen is a +2 enhancement that improves the threat range of a weapon by 1 rather than doubling it and Improved Critical improves the threat range of a weapon by 1 rather than doubling it. Heavy fortification armor doesn't exist. Light fortification is a +2 bonus and medium fortification is a +4 bonus.
Without these motifications, I believe the system would be dramatically more deadly than a normal D&D game. I also believe it would be dramatically unbalanced in a number of ways:
1. Too much emphasis on con. If con and toughness are the only ways to gain wounds, a character with a higher con will always be dramatically better than one with a low con. Adding a character's level to wounds means that Toughness is still important and con is still VERY important but it slightly ameliorates the importance of con.
2. With power attack and mid-high level characters, the all-damage goes to wounds system is just plain unworkable. The 12th-14th level normal D&D characters I play or play with regularly do +20 damage on a normal hit (it's not that hard, my 15th level fighter/wizard usually deals 2d4+9 (+6 str, +3 enhancement) plus 1d6+12 fire from Fires of Purity on a normal hit; give him a polymorph or Enlarge Person spell and that goes up dramatically). 8th level half-orc fighter/barbarians often do 2d6+13 points of damage/hit without power attacking but since they usually power attack, make that 2d6+17 to 2d6+29 depending upon how much they power attack. Quite simply, as soon as power-attacking characters leave low levels, combats will just be a matter of "I crit, you die." Limiting the wound damage to base weapon dice solved this problem neatly, making crits dramatic turning points in combat but keeping combats from simply being a matter of who crits first.
3. Related to #2. The D&D weapon system is balanced on the idea that 1 step of crit range is worth roughly 1 die size of damage. A falchion does 2d4 damage to a greatsword's 2d6 and has 1 point more crit range. A heavy pick or a scimitar does 1d6 points of damage to a longsword's 1d8 but has a crit rating one point better. Mathematically, there are advantages and disadvantages to all the kinds of weapons but generally which you choose is a matter of style and preference. They're roughly similar. Not so with the wound/vitality crit system. Since die based damage is not incredibly significant past level 3-4 (at which point, strength, power attack, weapon specialization, etc become more significant than the damage die) and crit=instakill with even minimal power attack, high threat range weapons are so dramatically and obviously better than other weapons, it's not funny. Given the choice between a falchion and a greatsword, that extra 5% chance of killing your foe per swing is FAR better than 2 points of damage. Again, limiting the wound damage to the damage die made this a bit more balanced. Higher damage die weapons are worth considering for the extra wounds they do on crits. 2d6 wounds 33% less often is at least worth considering compared to 2d4 wounds on a more regular basis.
Star Wars doesn't have this problem because all the weapons are balanced with wound/vitality points in mind. (And, IIRC, the only weapon with better than a 20 crit range is the lightsaber--which is supposed to be the game's ueberweapon).
4. The system makes fort saves DRAMATICALLY more important. Every time you're critted, you make a fort save to stay in the fight. In order to avoid dying, you make a fort save. To stabilize you make a fort save. Etc. etc. I reduced the stun duration to 1 round from 1d4 rounds simply because, IME very few combats last more than 4 rounds and the 1d4 rounds of stun would remove characters from combat completely. At one round, fort saves are still dramatically more important than in normal D&D but it's conceivable that a character might not have great fortitude. Being stunned for 1 round is still REALLY nasty since you drop your weapons and your foes get a free round to beat on you.
5. Without vitality points, there's no point in using warriors as adversaries for the PCs. Even if they survive a blow, they're going to be stunned and fatigued. No-vitality storm troopers may be good for Star Wars where characters are supposed to mow them down by the dozen but for a D&D game where I want the town militia to have a hope of defending their town, where I don't want to make the town guard all fighters just to keep the PCs from killing them all, and I want the orc tribe to be respectable warriors without making them all barbarians, it just wouldn't work. Since I want NPC classed characters from orc warriors to noble aristocrats to be potentially serious foes, they have to get vitality points.
With the modifications, the system is about on a par with an ordinary D&D game but magical healing is somewhat less necessary (since characters can recover vitality points quickly between encounters). Characters will hit the dirt more often due to crits and the stunned death spiral but not having the -10 you're dead cap and making fort saves rather than stabilization rolls means that they're less likely to die.
As written, I would say that the system is FAR more deadly than normal D&D. Any foe with a good weapon and power attack will drop a PC 3-6 times out of 20 (depending upon whether or not they have improved crit or a keen weapon). Given the number of fights PCs have to survive, regular TPKs are almost inevitable. (I know I would have had a TPK in the fight where I rolled four consecutive crits with the griffon; using my rules, I took two PCs to death's door and wounded a third instead).
