There are so many different variations on the push-up, that if you do it smart you can actually see great gains. Simply doing 100-200 regular push-ups isn't going to do much though.
It depends on your base level and what you are trying to accomplish. If you are just trying to get some exercise into your routine and just use it to move around now and again, you'll be fine. Adding a little variety into the mix like inclining your feet, and changing hand position will keep working different parts of your arms. Another good variation is the "hindu push-up". Do a Google search until you find a photo (should be easy to find). That variation really kicks your butt the first few weeks. Doing this will give you small gains in mass, and great gains in endurance. The fewer the reps you can do, the more mass you'll gain - the more reps, the more endurance.
Once you get used that that routine, you'll eventually hit a wall if you are training for mass. Well, then you start adding harder variations. One handed push-ups, inclined one handed push-ups, hand stand push-ups, etc. As long as you can only do 3-5 reps per set, you're building mass.
Once you can do one handed hand stand push-ups though, you'd better buy some weights
Obviously, the benifits will be weighed against what you want. If you want to be a world-class weightlifter, you need to lift weights. Modified push-ups and other calithenics are used by professional fighters who need both strenghth and endurance. In my opinion, the difference in ability is only slight, and a fighters training tends to be a little more healthy. Not to mention cheaper.