This might be one of those apples/oranges question, but I am a novice in photography and I recently played around with Canon EF 70-200mm f/4 L IS USM and loved its picture quality (especially the colors in outdoors). Now i don't expect this lens to create the same type of shots, but how close will it be? Should i just spend more on the L series or get this as a placement for now?
First of all, we don't know what body you're using it on, but I don't automatically assume it was a full frame body, since the 70-200L works great on crop bodies too.
Anyhow, the two lenses are indeed apples to oranges, and not only because of the quality and price, but mostly because of the intended use.
The 18-250 OS is a decent one-lens solution, actually considered probably the best of its kind. If you are thinking 70-200L you absolutely need to have something in the normal zoom range too (for example 18-55 for crop bodies, or 24-70 or so for full frame bodies.) If you have that already covered that you should be fine, the 70-200L (both IS and non-IS) is a great lens. But of course the non-IS is around $450-600 used, the IS is around $900-1000 used.
So basically this Sigma is for people who would like a decent one-lens solution for hiking, walkaround, travel light. If you have a 70-200, then you most likely need to take a standard zoom with you too, and those two lenses plus body are not small and light for my definition of light travel/walkaround gear.