OS will only boot up faster with a solid-state drive. If you perform any operating system related task that needs to retrieve data from the drive, it will be much faster rather than the HDD. That being said, if you only care about loading subsequent programs (or logistically prefer keeping your OS separate), you can easily keep the SSD as a secondary drive, only using it for certain programs/tasks.

The whole point of a solid-state drive is to decrease application loading times. This is more due to the lower seek time rather than the faster transfer rate, which makes it more like RAM. In fact, some users would be better off getting more RAM than a solid-state drive - but that always depends on your needs.

The main storage device (SSD or HDD) is always the bottleneck of any computer system. While SSDs help to alleviate this bottleneck, new ones are still only ~1/40th the speed of RAM. For example, some memory bandwidth in newer computers has reached over 20,000 MB/s versus some new SSDs which top out at just over 500 MB/s.

You can also use it for the increased sustained transfer speed, but that only applies if you deal with very large file transfers with, for example, video encoding.

For the fastest experience with your computer, install your OS on the solid-state drive, but do remember to make frequent backups.

Hope you find this information useful.