Hey Suhaimig,
Think you would have gotten almost instant replies had you posted under the section on Nikon Equipment.
I use an MB-15 with my F100 like many Nikon users out there. A new one will cost around S$280 (may have gone up with the increase in GST). I got mine used in a mint condition at The Camera Workshop for $200, which is a reasonable price to pay.
There definitely are differences when using your F100 with and without an MB-15.
One of the most obvious ones is the change in size of your grip, of course. It'll also be much easier to shoot portrait with a long lens. The main command dial and AF-lock on the MB-15 are very useful.
Also the number of frames/sec you can shoot increases. Don't have the figures off hand but I'm quite sure you get the same fire rate as the F5.
On the other hand, your camera gets severely heavier. Battery usage also increases to 6AAs.
Keeping this brief which is why I only mention the gist. To find out more, just walk into any store and try out for yourself.