Have 2 rifles with forward mounted Leopold IER Optics - one fixed 2.5x and the other 1.5-5x.
Pluses: Huge improvement over open sights at closer ranges; Forward mounting eliminates any potential scope biting of your face; Cool as hell; Forward position may improve rifle balance; Cool as hell to use.
Minuses: Much smaller Field of view; limited magnification capabilities.
Opinion: Great at short, intermediate ranges. Perhaps a little faster than a standard mounted optic. Allows 'situational awareness', ie. your view isn't blocked by being so close to the scope.
However......I find a 30mm 1-4x or 1-6x receiver mounted gives great field of view at all ranges, is very fast to use especially at lower power.
So, in summary, they work great at shorter / intermediate ranges, are cool as hell and an improvement over open sights. Won't be as precision capable as receiver mounted optics.