Well, I use my 300 for my primary wildlife lens. With a 2x TC, its a hand holdable 600mm 5.6 with a very short minimum focus distance so its great for photos of small passerines, shorebirds or shooting from a hide. It's easy enough to hike with once you get used to it especially with a nice black rapid style strap. I've also used it for field sports with and without the 1.4x TC. Its good for tight portraiture, especially for indoor sports where you will definitely need the wide aperture.
There's plenty of good uses for the lens if you shoot the style of photography it calls for. Mine is the ancient non-IS version and I would instantly trade up for the vII if I could afford it. I bought the sigmonster because I got a crazy good deal on it and it will come in very handy for when I'm distance limited like, say, shooting waterfowl or in a restricted habitat but I will almost certainly continue to use the 300 primarily. It is just too good and too useful for wildlife and sports. I'd say keep it around for a bit and see if you use it. You can't really lose too much money if you find yourself selling it on later and you'll never be hard pressed to find a buyer. I'd guess that if you have any interest in wildlife or sports photography, you won't ever want to get rid of it.