Great posts above.
Yeah, anything can be abused, with the consequences of that particular substance being relative to it's nature as a substance and it's specific interaction with a human ; all substances varying in their side effects and withdrawal effects.
For me, i've worked with DMT since January of 09', probably close to a couple hundred journeys, probably half of those being breakthrough/full dissolution types of experiences. I smoked DMT/changa alot during the first year or so, half of that first year being 4-6 times a week, eventually dwindling down to 2-3 times a week for that first year. Many call this phase a 'honeymoon' phase. For me, I 'really' wanted to see what this experience could do, so I partook often. Eventually, over the years, with the experiences i've had, I rarely touch it now; maybe 3-4 times a year, maybe. The sheer power of this experience is beyond what we can grasp; it's humbled me to the ends of the earth; and with that - I keep a healthy respect knowing it's potential to rip my life apart, to immediately put into question everything I thought I had an understanding of. These are my views though, so I realize that they don't apply with everyone.
With this route though (dosing consistently), sooner or later, a hyperslap (difficult experience) is typically in store. Those can do a wide range of things as far as after effects though - you could swear off the experience for a huge amount of time, it could intrigue you more, it could humble you to the n'th degree, you might quit DMT altogether; it's really hard to say specifically. For me, there were a few experiences that were so overwhelmingly deep and powerful early on, that i took 5-6 months off; swearing off DMT for a long while. The DMT experience is pretty self-regulating if you're working with committed dosages and are fairly frequent in dosing - it's typically just a matter of time in that case that the self-regulation takes effect.
Could the experience be abused? Sure, if you're staying in threshold levels and using these threshold levels in a range of recreational activities (or even just sitting around on your couch). I could see how you could get addicted to the various effects at those lower dosages.
There's a hump you get over with the bigger dosages though, especially when they're efficiently delivered. More times than not - at that degree of dissolution and breakthrough; that's typically not something that most are so keen to repeat quickly; typically those types of experiences satisfy for a decent amount of time.
For me personally, there hasn't been any negative effects i can think of. As Metanoia said, it's all been mostly positive. Life/reality is visually more beautiful; the smallest things tend to move me many degrees moreso than before. Mentally speaking, my mind is more impartial to most things; I rarely get worked up over much, thoughts I can usually just observe and let go of; not holding onto things like I used to. (im not perfect though) Psychedelics, especially DMT, have opened me up to this pervading calmness; something I can't really put my finger on - something that seems to pervade all of reality, something that I feel intensely a part of, inseparable. Most of times, I can get out of my thoughts and just abide in that calmness; a sort of detachment I guess; yet highly aware of everything. Life's became wordless; always feels like a Mystery; that childlike wonder pervades my life; alot of the time I get lost in it. It's like my imagination is running on high octane fuel.
**That all being said - we're all human, geographically we are all spread out; different cultures, different ways of life, different upbringings, different hopes/fears/loves/memories so on n' so forth. All these various life factors have shaped us, more or less, as to how we perceive and take in and react to Life and all it's various experiences. Psychedelics imo do what they do; they're there as a tool for the Self; and how we choose to react and assimilate these experiences falls on us. That being said - these experience can change our life for the better, or they can change our life for the worse; all resting on the individual and how they choose to react, assimilate , and mold the realizations into their Life. I think a healthy respect is important. These experiences, especially the ones that completely obliterate your life and everything knowable, 'can' drive you mad if you allow it; often it's a very fine line to walk. With that - as many here have attested including myself - having daily activities in which you can funnel these internal energies/thoughts into - whether it be yoga, some form of meditation, art, exercise, writing; or even just walking in the woods - these all can greatly enhance your relationship with these experiences, your thoughts and Life.