FW is a very high damage spell. Although 1.13 gave it synergy bonuses from Warmth and Inferno, it doesn't really need them. For this reason, it's a good "backup" skill. You can spend 20 points in FW and 1 in Fire Mastery (FM) to get decent damage to deal with immunes to your primary element, especially given some +skills.
I'm still maxing my FM, so FW hasn't even reached its full damage potential yet. It still does around 3,000 displayed fire damage per second with minimal +skills, which is quite sufficient for late Nightmare where my character currently sits. I've seen people state figures of 7,000 FW damage, which doesn't surprise me at all. I have a +3 FW Leaf staff that I'm not using anymore. The damage and range of FW go up dramatically when I pop that thing on. You can shop these from Drognan in Act 2. A +3 Fire Wall Leaf staff is an excellent twink staff if you are waiting on level 30 skills.
FW also has a long timer (1.4 seconds - Meteor is 1.2, Blizzard is 1.8 for some comparisons). So, you need to have good FW placement in order to make the spell effective. If you miss all the monsters with your first casting, you have to wait 1.4 seconds to cast again. That's a long time to let monsters go unharmed.
Contrary to what some people may think, you could call FW a ranged spell. The skill description displays that it is 30 yards long (so 15 yards on each side from the point where you click) at level 20 (although it is actually 2 2/3 yards longer than displayed). Add +skills and this figure keeps on going. There are ways to hit monsters well off the screen using this spell as you'll see in the screenies (they're coming I promise I'll stop talking soon).
I picked a Holy Freeze merc. It's the logical choice for my particular build (FW/Nova). Barb mercs have knockback, so they can knock monsters out of your carefully placed FW. Thus, I recommend the Holy Freeze merc or an A3 Cold merc. The A3 merc can freeze them in place while you fry them. However, most sorcs prefer (or need) a tank. So, Holy Freeze is the best choice in my mind.
It can be a little difficult to master/control. You may be really good at using FW in solo games, but party play can move quickly, so you may struggle to use it well in big parties.
There are also some monsters that you won't be able to hit with Fire Wall. The Lurkers that surface from pools are one example. Any monster that can fly/hover over water will be unreachable, also. (Succubi in Frozen River, Suckers in Act 3).
How FW works
First of all, you must remember that your character always stays in the middle of the screen. Also note that you need an uninterrupted line-of-sight to the casting point of the FW.
Ok, there are two relevant lines that we are going to discuss. The first is the "character line." The second is the "FW line." The character line extends from your character (the middle of the screen) to the point wherever you click to cast FW. The FW line then extends perpendicularly from the click-point.
The character line and the FW line will always be perpendicular to each other.
So, with this knowledge, you can aim your Fire Walls. In these two shots, I want a vertical FW.
Here are two screenies that show how to position a FW so that it "blocks" a doorway and continues through the room.
Durance hallway example
This is a good spot to show how I proceed through hallway-type places. My technique usually does a good job of clearing things out before I have to move in myself.
These next four screenies show how to kill monsters safely from around a corner. I've added red lines to show the line from me to the point where I clicked. Then I added a perpendicular line that goes with the FW itself. Circles are points of interest - usually dead monsters.