I tried a two-mode E2L, and I didn't like it. I think it's terrible for tactical use. From what I understand, in a tactical situation, you won't be using your light in a 'constant on' mode. You'll have it in your hands and when you need to light something up, you do it for the absolute minimum amount of time necessary. This usually involves a lot of intermittant clicking. No problem so far.
The problem with the Surefire 2-mode clicky lights (E1B, E2DL, etc) is that if you're frequently clicking it on and off, the mode will constantly be changing, since the mode changes if you click the light on within a certain time frame (a couple of seconds?). Thus, you'll be in a situation where you want all hi-mode, and everytime you turn it on you'll get hi-lo-hi-lo-hi. Or, you'll be in a situation where you want lo-mode, and you have to turn it on high first to get to lo. The trick is to cover the lens with your hand until the light is at the right level. But you can't do this if you have a weapon in your other hand.
I prefer lights that have non-switch selectable levels. My current favorite is a Surefire U2. The best thing about it is that it has both a clicky and selector ring to allow the light level to be selected independently from the switch. Also, it has a very low lo-mode. This is great for walking around at night where you want to see the ground but don't want to ruin your night vision.
Although the U2 is great, I think a better, cheaper alternative is a Malkoff MD2 flashlight with a hi-lo ring. The hi-lo ring allows the user to change the output levels by twisting the bezel. The Malkoff dropin is simply the best in the world, bar none. Plus, you can easily upgrade your light as new technology comes out. The price is cheap, in my opinion, for what you get.
Good luck, and stay safe.
-John