Are push-ups still considered an effective form of exercise?
Push Ups are a great body weight exercise. They put a lot of your core muscles at work. Doing push ups won't get you huge, it will get you fit (assuming you eat properly ando some other exercises too).
If some one tells you they are not worth it, it's because they don't do push ups, or they just do a few. If you want to grow a little and become stronger, you can do push ups 2–3 times a week, but have to do lots of them, it's actually easier than you would think. Aim at 200–300 push ups, in series of no less than 20 if you are beginning, or 30–40 if you can.
Don't forget that in push ups, as with any other body weight exercise, FORM is the most important aspect, a badly executed push up is not good for anything, it isn't maybe half a push up. If you can't do a nearly perfect form push up, first concentrate on doing them properly, once you can do 10 push ups in perfect form, you will be able to increase the number quickly.
If you can't do a lot of them, you can do them every day, always in perfect form, until you can do at least 30 push ups in one try. Then do another series until you can do no more. Once you can do 30–20–15 or so, you are on your way. If you reach 30–30–30 (or more than 50 in one try) you can change to 3 times a week. If you reach 250 push ups, you can move to 2 times a week trying to do more in sets of 30–40 at least.