Even though these days it's not usually necessary to compress files to save disk space, this utility can be used for more than just saving a few hundred megabytes. Splitting a huge file across several DVDs, encrypting a PHP project for safe keeping or grouping thousands of files in one single file are just a few things you can do with this program.
It's not the most powerful archiver but it's one of the most popular for a simple reason: the algorithm is fast and very efficient, considering its speed. Other formats offer a far better compression ratio but they require a huge amount of time to finish the job.
WinRAR can open a lot of archive formats but can only compress to RAR or ZIP. Recently, the new RAR5 format has been defined which promises a slightly better compression ratio, mainly because it supports larger dictionaries. Also, the latest version has added support for opening ZIPX files and 7z (7-zip) split archives.
The interface has been altered a little bit too in the latest releases. Encrypting RAR files with a password wasn't very straightforward in the past but now that this option has been moved to the main tab in the archiving dialog it's very easy to spot and use.
The default settings give satisfactory results but if you're a power-user, you can tweak more advanced options like the dictionary size, recovery record and more.
As it is the case with almost every mature and popular program, you probably won't find any problems. However, if you're an occasional user, the price for the full version might seem just a little bit too high. But you can test it out for 40 days and decide if it's worth every dollar.