Generally speaking, here's what I've run into:
HP/MDT: People who take little bits of damage over the course of the day several times end up being low on hit points, which is kind of a bummer when a real fight rolls around. In a game where I as the GM wanted something more realistic, it was tough to come up with reasonable areas of injury -- I could do it, but it took work. And there was always the "Okay, you are now 18 seconds from death... okay, now you're 12 seconds from death... okay, now you're -- wait, you made your save. Okay, you were six seconds from death, but if Ol' Doc can stitch you up, you will be completely and totally injury-free in all respects inside of a week." I was trying to run an X-Files-style game, and nobody ended up with casts or slings... which I sort of wanted. Not that I wanted to screw over my players. I just liked that flavor, but it was never warranted.
On the other hand, unlike many, I enjoyed the MDT when it came to nonlethal combat. My investigators could slug people, grapple people, whatever, and never be in danger of really hurting folks unless they really tried to do so. A fight that included two agents, one amazingly tough and one trained in the martial arts, and three angry frat-boys, was both comical and fairly realistic, turning into an ugly, messy wrestling match in a hallway, right up to the point where the martial artist decided to start doing lethal damage and seriously slug people.
WP/VP: My experience is fairly limited, but I liked some of it -- the quick recovery of hit points that allowed me to model VP damage as near-misses and fatigue instead of light wounds was wonderful. There were times when the flavor-text was difficult -- when someone took VP damage from jumping off a building, we had to make an awning for his character to heroically bounce off of before landing on the hood of a car and groaning.
I don't like how VP/WP seems to result in the race to get the crit, but the problem is about equal to the MDT system's race to bypass the MDT and force the "save or go to -1" situation. Just based on flavor, I don't like games where th scimitar and rapier become the coolest weapons by default. I know that they might not actually be that much cooler, based on the lower damage and the fact that such lower damage might be soaked by armor, but if a rule causes players to think, "I should really use ____, unless I'm willing to take a less-good weapon for roleplaying reasons," regardless of whether or not that thought is accurate, then that's something that needs to be addressed.
With the MDT, I didn't see this as an issue as much -- d20 Modern weapons have enough tradeoffs (at least in the core books) that there's no one "best" weapon in all cases. There are some weapons that are obviously not as good, or that have only limited appeal (lousy but cheap, lousy but much smaller and easier to conceal, mediocre but can autofire, etc), but my players never found one weapon that was always the best weapon to use.
So fundamentally, I agree with folks who said that either system works well. Gut-level, if I were making a new campaign, I'd go with WP/VP if the campaign were low-magic or no-healing-magic and didn't have a good Surgery system. I'd go with MDT for a Modern campaign in which my characters weren't expected to be action heroes -- an X-Files campaign, an "if we get shot, we should hurt for awhile" game, that kind of thing. Of course, I'd also modify this by allowing someone to spend an Action point once they hit -10 to be "in a coma or suffering some kind of longer-than-one-week injury", so that I could better model the TV shows that fuel most of my campaign ideas -- where characters might get serious injuries (in a coma, broken arm, etc), but don't require a Treat Injury check (DC 30) by a Field Medic to save somebody who hits -10.
In an Action Hero game, where the hero should be able to whup a whole bunch of bad guys, even getting hit along the way, and still be fresh as a daisy when he runs into the end bad guy, I'd use WP/VP, since it allows for faster recovery times. Or I'd use the M&M injury system, which goes something like this, since you asked earlier:
(Note: I'm describing how this works in M&M, not how it works in Unearthed Arcana -- Unearthed Arcana is a bit of a kludge this way, in my opinion, and it's better to just wholly transition over to the M&M damage system if you're doing to use this type of mechanic)
-All attacks do a set amount of damage. A Longsword does +4 Lethal, 19-20 crit. A baseball bat does +3 Stun. A chainsaw does +6 Lethal. A rapier does +3 Lethal, 18-20 crit. You add your Strength bonus to damage like normal.
-Armor soaks damage. A chain shirt might soak 3, while plate might soak 7.
-Whenever somebody is hit, they make a Damage Save (which is a Con-based save that is like a Fort save but applies only to damage -- the Fort save stays around for poisons, diseases, and other stuff like that). The Save DC is 15 + (damage dealt to them). If the damage is reduced below 0, they don't need to make a save -- so good armor gets you out of a lot of save attempts.
-If you make the save, all is well. If you fail the save by 1-5, you take a Hit -- which means a -1 penalty on all saves. If you fail the save by 6-10, you take a Hit and are Stunned for one round. If you fail the save by 11 or more, you are unconscious.
The result of this system actually mirrors hit points or vitality points fairly well, and requires less math. People ignore a lot of hits, but as those penalties start to creep up (2 hits gives you a -2, 3 hits brings it to -3, 4 hits bumps it to -4), they come closer and closer to failing saves by larger amounts and being rendered unconscious.
It's a good system -- I've liked it a bunch as I've used it, and I've used it in both Superhero games and fantasy games -- but it requires a lot of fuzzy math or a conversion that makes your d20 Modern or D&D books less useful.
Really, any of these systems can be good. It all depends on the game you want to run.
