I've found that melee weapons are perfectly capable in interiors, where you can spring upon your enemy from around corners like a lawnmower of gory decapitation, and you can also limit the amount of enemies simultaneously trying to gun you down. Outdoors or out in the open, however, you'll have to shoot yourself up with more drugs than you can shake a stick at to be effective, since you tend to be constantly surrounded by foes and deprived of effective cover. I don't play a melee 'exclusive' kind of character, but I always carry a light melee weapons to help conserve ammo when dealing with easy foes and also to deal with specific threats, such as Sentry Bots that can otherwise soak up a lot of bullets. What my limited meleeing experience has told me is this:
Always approach enemies by moving from cover to cover and encourage enemies to empty their clips while their chances of hitting you are slim, forcing them to reload before you rush in for the kill. Running into a hail of bullets is never a good idea, especially not when a lucky series of critical hits can leave you stun-locked.
Having the Tackle Perk helps when you decide to charge the foe but Bullet Time can also assist in allowing you to get a couple of good strikes to their arm while they're reloading. If you have decent melee skills and luck, you're prone to causing a critical hit or two, probably crippling the enemy's arm and making them drop their weapons. Striking the head might do more damage, but the arm's easier to hit- and without their gun they're pretty much done for anyways. :toughninja:
Some foes can take a lot of punishment and also dish out a lot of it, such as the Sentry Bot, and can be a real hassle to deal with even for a gun-wielding character. Should you get up close and circle the enemy you'll find that they often have issues keeping up, and struggle to turn to face the right direction. This gives you a perfect opportunity to give them a good thrashing with your melee weapons, often allowing you to defeat them without being hurt at all. Humanoid foes are usually more capable of resisting this tactic, but remaining mobile and 'dancing' around them still improves your chances of getting through it alive.
I suppose this cover-to-cover, ambushing and circular dancing approach doesn't exactly scream "Badarse Brotherhood Warrior" but it's undeniably more effecting than rushing right in there without a plan, especially early on when you won't have power armour or chems to aid you.