The practitioner needs to attain the form jhanas first to get to the formless (at least initially). Through 1 to 4 form jhanas there is an increase in one-pointedness of the mind which needs to be developed. However in the formless jhanas the one-pointedness level is as the same as in the fourth form jhana (there is no gradual increase). So unless a person can tap into this level of unification of mind it is unlikely they are going into formless jhana. So IMO practice of form jhanas (which is difficult enough) must be performed before formless jhanas are attempted. It makes sense to get to the fourth jhana and then try for the 5th (ie -formless) jhana.
There is a main experiential difference between the form and formless jhanas. The one-pointedness of mind in the form jhanas are inward directed. In the formless jhanas it is out ward directed (Hence descriptions such as infinite space, consciousness etc). While these can be used as objects (using the 4th jhana as a base) it maybe easier to intentionally focus on the breath (which you can do) and this will take you to the 5th jhana, and so on for the higher jhana.
I find that one of the main advantages of formless jhana is understanding that increasingly higher levels of peacefulness is what we are searching for in the path. At the 8th jhana there is still some arising and passing away- hence there is unsatisfactoriness even in that. By seeing arising and passing away constantly (hours,days- when doing vipassana) we become able to let go of even that little bit of phenomena- hence reaching a perfect peace of nibbaana. It is said in the suttas that even the first jhana is enough to become an arahanth. There is probably no extra advantage in going beyond the fourth jhana in terms of it helping the mind through samadhi/suppression of hindrances.
Another thing that comes to mind is that mastery of the rupa jhanas has been praised by the Buddha.