As mentioned above Dryject is a great tool but not a complete replacement for aeration.
The sand injection has many benefits and uses.
It is a fantastic method to help convert a soil based green over to sand. By pulling cores and injecting the sand, over time, the rootzone will become more sandy and drain better. The other methods such as drill and fill work well too but as Pat mentioned DJ is a lot faster, cleaner and cheaper. The depth can be varied also which helps with this process.
It can add soil amendments (for nutrition holding) to sand based greens.
It will dilute organic matter in the profile by essentially blasting it and spreading it around deeper in the rootzone. While this might sound like a negative, we are talking about small amounts so they can decay without causing an issue. I’ve seen OM at less than 1% but the breakdown of the organic has made the upper sand darker due to the remnants of the carbon from the breakdown. This can also impede drainage and the DJ will move this soil though the profile - this is the main reason I use it.
The blast creates drainage channels in the thatch/mat layer that helps with surface drainage (provided the water has somewhere to go).
It can also help break up compaction layers in the soil as it blasts its way through.
It also firms up greens, which brings us to johns comment about adding compaction. By forcing more material in, it will increase bulk density, hence why other aeration practices should be used to mitigate it. I get that that sounds like a contradiction and creating more work but it will take a long time for the DJ alone to have a truly negative effect on bulk density. If you’re using Dryject, it is to accomplish some of, if not all of the items above so core aeration will still be carried out anyway. The Dryject is an add-on to accomplish other things.
I have also contracted them to just push water through the profile like the old Toro Hydrojects. In summer this is a great addition to needle tine aeration as it is non disruptive and goes a lot deeper than the little tines. It also helps me move that om carbon layer through the profile without adding any material. Just be careful in the heat of the summer to nor scald the roots.
I grew the greens in at LedgeRock so there would not be a grow-in layer and this has been greatly beneficial as the club didn’t have to work to get rid of it later. I have had the greens physical properties tested twice a year since then too to ensure the OM is where it should be. So in over 10 years, I have proof that the Dryject has not increased bulk density or created more compaction - there are no negative long term effects, only long term benefits provided it’s used properly in a program.
One big word of caution (although this also goes for any type of cultural practice that adds sand) is to make sure the water has somewhere to go. Dryjecting will convert a soil over to sand but at the bottom it will create a perched water table with the native soil so the sand will hold water (you’re essentially creating a bathtub). Therefore some sort of internal drainage is essential to get the water out.
A number of clubs have done Dryject on fairways to increase drainage and I know that Hamilton Farms does it at least yearly. At the very least possibly using it in wet areas on fairway would be a cost effective solution to drainage problems.