Yes. Condoms are incredibly safe, when they are used properly. In most studies, they have been found to be almost as effective as birth control pills, with most showing a 98% or greater effectiveness, when used correctly. Meaning, if 100 women use condoms for a year, only 2 will experience a pregnancy.
So, are condoms safe enough to prevent pregnancy? If you use them correctly, it's highly unlikely that you will become pregnant. However, many people use them incorrectly, sparingly, or use the wrong kind.
Studies which look at standard condom usage have shown considerably less effective results, although still over a 80% prevention rate. However, it's important to note that these studies include people who simply didn't use the condoms, or used them incorrectly.
It's important to recognize that condoms are one of the few contraceptions which can also help to protect against STDs (sexually transmitted diseases), which is why they are highly recommended.
Yes, they are. But you must use them correctly. Many people don't put them on the right way, they use the wrong size, store them in terrible conditions, use them past their use-by date and let them slip off.