Deciding to install in-ground hot tub is not a minor choice. Once it's done, you're more or less stuck with where it is, unless you're willing to pay a pretty hefty fee later on to have it dug up and moved. Having said that, getting an in-ground spa is appropriate for some people. Ultimately, several factors come into play as you evaluate the relative pros and cons of choosing a permanent hot tub.
If space is strictly limited in terms of where you can feasibly put a hot tub, then an in-ground model may be a good choice. While you could also choose an above-ground hot tub in this instance, in-ground tubs often look sleeker and less cumbersome. An above-ground spa might also block the view from inside your home or present some other obstruction that would prevent it from being used.
An in-ground hot tub is easier to access. Apart from the day-to-day convenience, you or a family member might have a condition or injury that would make it difficult of impossible to climb steps to get into your hot tub. In-ground tubs eliminate this process and may be the viable option for you.
In-ground hot tubs can be more energy efficient. While this is becoming increasingly less of a concern because modern portable spas are built with these factors in mind, the heat loss from an in-ground model still tends to be lower. If you live somewhere that gets cold in the winter, this can make a big difference to your winter energy bills.
Probably the biggest dis-incentive to installing an in-ground hot tub is the lack of portability. You'll have to live with the location that you've selected once the permanent spa has been installed. For some, this is not a major factor (you may really only have one location that is suitable). If it is a point to ponder, ask your local professional pool and spa dealers for advice. They'll be happy to help you make the best decision for your situation.