vikram1988...I bought the Sigma 120-400 f/4.5-5.6 OS. I like it a lot. The "OS" works well, but seems to be flawed (on my copy anyway...perhaps on all copies). It imparts softness, even though it is stabilizing up to 4 stops. It is usable only if your final image will be printed 5x7 or smaller...or 8x10 if you don't mind a slight softness.
With it switched off, the lens gets extremely sharp in the center at 400mm, especially at distances closer than 100 feet...even wide open at f/5.6. Closed to f/9, it's sharp to the image borders on both full frame and a crop sensor, at 400mm...at infinity focus, and everywhere else. Closed to f/11, it's razor sharp even on a full frame, to the corners, throughout the focal range. At the wide end, it's sharp to the full frame corners, by f/6.3. However, it's sharp to the full frame borders even wide open at f/4.5...at 120mm.
Despite much negativity heaped on this lens and its supposedly "sluggish, inconsistent autofocus"...I have found that not to be the case, with my copy. It focuses very fast and accurate, even in low light in servo mode. For just over half the price of the Canon 100-400, I say it's a no brainer. Color and contrast via this Sigma, are at least on par with, if not superior to the Canon...up to about 250mm. Beyond that, it's possible the contrast and color saturation are slightly less than provided by the Canon...but the Sigma color balance is warmer, which I much prefer.
I've rented the 400 f/5.6L, it's vastly sharper than either of the above zooms. However, I suggest if you must have a Canon lens, forget the 100-400, and the 400 prime, and instead go with the 300 f/4L, plus a 1.4x TC. It will give you very decent image stabilization, light weight, compact size, much faster aperture, nice bokeh, and almost the sharpness of the 400 f/5.6L prime (at 300mm), and perhaps a bit more sharpness at 420mm f/5.6 (via even the older version 2 1.4x TC), than the 100-400L at 400mm...but with an extra 20mm thrown in.
If you simply must have a walk around zoom, budget for the 70-300L. If you want a vastly better lens than your 55-250, with slightly less reach for not much more money, try the 70-200 f/4L (non-IS)...and save a while longer for a full frame, or a 70D, or a 7D2.