I would really recommend this combo. I tried it once about a month ago and found great synergy between the two. I found it to be very psychedelic, approximately on-par with a normal dose of psilocybe mushrooms (but with slightly different effects of course). As a disclaimer though, Cannabis is usually very psychedelic for me (with a lot of visuals and whatnot), so this combo may not be psychedelic for you if you have never experienced this with cannabis.

I took 250mg 5-htp, waited about an hour, and first smoked a slightly below normal dose of cannabis. I then quickly realized that the effects were different and a lot more psychedelic than normal, so I decided to smoke more to see how far I could go. This led to a 3-hour trip that was far more intense and psychedelic than I ever expected would be possible from just weed alone. Although I normally hallucinate from cannabis, I found the effects from this combo to be more reminiscent of a serotonergic psychedelic than cannabis alone (which isn't surprising, considering 5-htp increases serotonin levels).

Has anyone else tried this? If so, did you experience something similar?

5-HTP is the precursor to serotonin(5-HT) and after ingestion it's quite metabolized. Shulgin did some research by just taking Tryptamine as can be read in TiKHAL and it appears to be psychoactive as well.

It could be that serotonin in itself is psychoactive or that high levels of serotonin potentiate the effects of the cannabis. It's hard to determine without further research, but interesting things!

Please take the usual precautions when taking 5-HTP that are to be taking when taking harmalas. Combining the two might potentially lead to serotonin syndrome.