I went from side drafts to megasquirt on my daily driver. So here goes
Yes I like it, much easier to tune (I was using a wideband 02 w/the carbs as well as MS), no more changing jets. But really you only go through that in the beginning and there are folks out there way way better at tuning carbs than me. My cabs would come out of sync occaisonally, they were used and tired, this doesn't happen now with the 318i manifold. A plus is the car doesn't smell like gas all of the time with MS. Was it worth it? That's sorta subjective, I get a little better fuel efficiency now. I like tinkering with the 02 (carbs) and taking on new projects (MS), so that factored in to it for me. I also thought I may do a turbo at some point, though now I'm leaning m20, might remain a daydream, but whatever. I wanted to become more familiar with EFI. The MS is well supported on their forum and plenty of folks here have done it, so you can usually reach out to get questions answered.
I don't have any HP or torque #'s since I have never done any dyno runs. I have a rebuilt motor ~9.5:1 comp and a schrick 292 cam. The car really came alive above ~3k with the carbs, with MS it still does but seems to be a little more responsive off idle.
I don't remember what the cost was. I collected most of the EFI bits over time as I could. I bought the MS stuff from DIY autotune and miscellaneous stuff off craigslist and ebay (EDIS stuff mostly). It was probably about the cost of a new side draft setup but more than the used set of cabs the MS replaced. Also because there are many ways to install MS costs will vary.
I would say not too complicated if you are already your own mechanic in most regards. I had never built any sort of electronics before the MS and computers were a way of accessing the internet. I think it took me a little over a week to do the install working in the evenings (not including building the MS stuff), some may work faster. I would highly recommend that you have a plan and have everything on hand before you start, I can't stress that enough. Nothing like waiting for some hard to source part to arrive, or make several trips to the salvage yard in order to move on to the next step.
The down side is when I open the hood now my car doesn't look nearly as bad ass as it did with the webers. If someone were starting from scratch I would recommend MS for ease of use and webers for period correctness etc.. I found them both equally drive-able for everyday use with the MS offering a bit of an advantage in regards to initial setup, fuel efficiency and flexibility for future mods.