I have IVed DMT on a few occaisions. It is totally comparable in effects to smoking IME, the onset is basically the same (maybe slightly more rapid IVed but inconsequential), duration is also comparable. Of course, dosages need to be adjusted down but I think this is due to losses when smoking and not majorly altered metabolism of the drug when smoking (conjecture). The effects are basically the same but it is 'simpler' to break-through when IVing.
Of course, you need to understand that you are not going to be able to do much about the needle or may find it taints a trip to some extent. I jabbed myself when on DMT after IVing (this during an extended few year period of IVing heroin/morphine) and this really weirded me out. I basically threw the needle away from me in horror and the experience was much murkier than I was used to. I haven't done this for many years and do not ever plan to again.
You also need to be quite sure of the source of your DMT. There are discussions about this somewhere, but I am (maybe incorrectly or overly) concerned about injecting plant matter and other unintended aspects of the material.
IVing psychedelics is no longer of interest to me. It was never something I did a lot of, oral dosing of most is more than adequate.