I've read through many of the posts on this forum, and it seems that, at one point or another, every single weapon has been argued to be overpowered, or underpowered (such as the lancer).
As such, i've decided to go around playing 19 round matches using only certain weapons. First test was the lancer. Each time I play as a non-host player.
Played a 19 rounds canal, used the lancer, the lancer, the lancer, and nothing but the lancer.
Turns out I got 36 kills. (Quite pleased with myself actually).
I believe the lancer to be the perfectly balanced weapon. It is a weapon that will win most mid/long range shootouts (exception to longshot...although i did kill a longshot at long range). You have to strike a balance between knowing when you have the shooting advantage in a shootout (ie, when to stand your ground), and when to dive back into cover. Aiming is obviously extremely important at this point.
In the short range it is still valid, as if you keep zigzagging away from a shotgun whilst spraying lancer fire at the enemy, you will eventually down him. This however, only works either if you are good at your movement, and aware of any objects around you that could block you for that crucial second, and, in most cases, if you have an active reload in there. Aiming at this point is a mistake, and pointless.
The lancer is also unique in that for one weapon it actually combines your good machinegun, with a chainsaw. One tactic I have found works extremely well (for some reason), is that where you keep spraying your close enemy for a few seconds, before moving in and chainsawing in a zigzag motion (ie, moving towards him suddenly and zigzagging). The idea is that after a few seconds of shooting, the enemy believes that is all you are doing and falls for the chainsaw trap (it does actually work).
A great use of the lancer is the camper ambush. The idea being that you hide behind a piece of cover with a line of sight to a hotly contested weapon. As the opponent comes to pick up the weapon, shoot. Sice he is immobilized due to the picking up animation, he is a sitting duck, which you take advantage of. By the time he has picked the weapon up, he is downed and yours to kill. Ideal examples are the boomshot in Gridlock, where you hide behind the cars and shoot through the passage, or the hammer of dawn in Fuel Depot, where you shoot from the sides.
Finally, the lancer is a time weapon. Unlike the AR sniper, boomshot, shotgun or AR pistol, it cannot down an opponent immediately/near-so. It takes at least 3 seconds to down the opponent, and that is if you have a quite good degree of accuracy. It does however, provide a very good anti-chainsaw weapon.
For the chainsaw, a working tactic that I use sometimes is as follows: When you are cornered by a guy with a shotgun, host or not, turn a corner (of a big box, or building), and then immediately turn around, run back around the corner and chainsaw your opponent (doesnt always work, but it sure has a high result percentage). The idea is that the noob will follow you around and get chomped. The average and experienced player will know you are *waiting* for him around the corner. However, they rarely expect you to actually run back around at them...
So, above was my evaluation of the lancer. I find it to be one of the weapons that is the most rewarding to kill with, and one that demands at least average skills to use properly.
Hope this proves insightful to newer players.