If I look at my statistics C4 has close to a 35% kill vs usage percentage for me. That includes moments when I cap a flag I will often leave a C4 surprise for the enemy if they try to take it back so not all of my "plants" go used. With grenades and throwing knifes I am much closer to 10% kill vs use ratio, that also does not factor all the doubles triples and quads I get with c4 which is way more likely than with grenades.

I am ranked in the top 3000 for the accolade for most C4 kills.

I was ranked a bit higher but I am working on the semtex, grenade, throwing knife and landmine lethal challenges right now. 

Here are my keys to success

1: learn the appropriate launch angle depending on your movement speed and angle of the throw the C4 pack will have different distances and trajectory as seen in my toss over the truck on Carbon. This takes the most getting used to but it is similar to learning when and where to throw grenades and semtex just not as much throwing range.

2: Learn popular camping spots and use recon/flashbang and radar to anticipate where enemys may be. Utilize this with #1 to learn to toss c4 into windows and buildings for easy kills. Stun grenades are also a big help for double kills+ lead with the stun makes it harder for them to scramble.

3:Watch this video and the others on this guys channel for great guids to C4 throwing spots and angles on different maps. I just recently found this after making one of the above videos and wish I found it sooner.

http://www.youtube.com/watch?v=_mRcpdUi_DU

4: Quickdraw pro plus double tap to detonate helps out greatly especially at the begining.

5: In objective game modes especially Domination plant your C4 and detonate when you hear losing A,B, or C