I was actually a bit of an acidhead when I was in my late teens-early 20's. I probably took LSD 20 or more times. My experiences with it were all over the map. I had some trips that were super fun, some that were highly uncomfortable, and some where I had deep realizations and experiences of being one with the Creator and all things. A lot of times it was a mix of all three. The thing about acid is that everything depends on your baseline psychological health level and the environment you take them in. If you are relatively happy and stress-free, with no major problems in your life, and you take it in a beautiful and comfortable environment with people who you love and trust, basically if you feel safe, then you are almost bound to have an overall positive experience (provided you don't take too much, that is - with psychedelics, less is more). But if you have major issues in your life, are in a stressful or overwhelming environment or are with unsupportive people, those become the focus of your trip.
On the other hand, in a sense every acid trip I took was beneficial in some way. It forces you to come to terms with the state of your life. It brings things to the forefront that your ego would rather keep buried. Acid trips last a very long time (up to 24 hours), that's a LONG time to reflect on aspects of your personality and your life that may be difficult to confront! That's why LSD can make people who are prone to certain types of mental illness go off the deep end - the realizations they have are too much for them to bear. When I was in my acid phase I was certainly not the epitome of mental health, but even the most uncomfortable experiences I had with it definitely yielded some sort of useful truth, even if it was just that all the people I hung out with back then were egotistical assholes.
Then there were the times that I sat and meditated for what seemed like hours, feeling totally in tune with my surroundings, with my true nature, with God. And times that I wept with sadness and joy at the same time, deeply feeling the plight of humanity and grieving for our collective "fall". And times when my friends and I had epic hacky sack sessions where we seemed to defy the laws of gravity (believe it or not, I was significantly better at hacking when I was on acid!).
For me, the big downsides with LSD were the extremely long duration of the experience, the shitty people I constantly seemed to be surrounded with, and.....this may be unique to me, because I've never found anyone else who had this issue, but especially towards the beginning of the trip I would get this awful sensation where I literally couldn't tell if I had to pee or not! I basically felt like I had to go all the time, but when I would go to the bathroom nothing would happen. It was very disconcerting and gave me a lot of anxiety because I felt like I was constantly on the verge of peeing my pants.
So I've also tried shrooms, Hawaiian baby woodrose seeds (which contain LSA), and salvia. I had nothing but bad experiences with all of these things. Shrooms always had a very "dark" feel to them that I just did not like. Thankfully they do not last anywhere near as long as LSD. I only took them a few times, and all of my trips were "bad". The woodrose seeds I only took once, and I took way too much! Interestingly, I had the same issue with the peeing thing, times 100! I had a horrible time which culminated in me puking all over myself. Salvia, for me, is the worst. More than any other plant or drug, salvia seems to have a personality, and it does NOT like me and I do NOT like it! It has almost a demonic vibe to it for me. A salvia trip is probably closer to what most people are imagining when they think of hallucinogenic drugs. I've done it twice, and both times the instant that I exhaled the smoke, I was like "crap, why did I just do that?" Fortunately the trip only lasts like 5-10 minutes, but time is all distorted on salvia. 5 minutes can seem like an hour. The experiences you have on salvia are nearly impossible to explain to others. The last time I did it, I literally saw AND felt all of the furniture in the room around me morphing into my body! I was running in circles trying to get away from it. Not cool. Then again, other people have totally different experiences with salvia - for some it is totally euphoric and pleasant. Definitely not for me though.
As far as any residual effects, flashbacks are definitely a myth. I've never had anything like that happen. I do know one person who took a lot of acid back in the 70's who claims acid gave him what he calls "eyelid movies" - when he closes his eyes, he sees images and scenes. He says they're not bothersome in any way, just entertaining. The only thing I could say I took with me from those experiences with acid is a sort of unshakeable optimism that I never had before. Before acid I was a pessimist. Life seemed pointless, and I spent my days mostly wallowing in self-loathing. Acid forced me to see the truth that it was a choice to live this way, and that God is real and the source of existence. It showed me the incredible beauty of the way all things fit together, even the things that appear "bad" from our subjective point of view. It's been almost seven years since I last took acid, and I have no desire to do it or any other psychedelic ever again. I feel they have nothing more to offer me. Sure, it can be fun, but taking acid is not primarily about having fun, and people who treat it as though that's what it's all about are bound to overdo it and have a traumatic experience. I would not reccomend acid to anyone unless I was sure that they were in the right state of mind and in a supportive environment. It's something that should not be taken lightly, but at the same time it can be an incredible therapeutic tool.