I'd studied Japanese fairly intensively for about a year before starting RtK. I had this really weird thing happen where I could read and recognize a fair amount of Kanji (recognize like... 600-700? this is a hard number to nail down, but a lot...) but write so very few of them. In fact, if I even tried to write them I would just sort of draw a huge blank - I would have a general image of what they looked like in my mind...kinda... but really unable to write them. And even when I did, I still wouldn't feel very confident in it at all. I would also have a great deal of trouble mixing up very similar characters, but obviously that never fully does go away.
Anyways, I completed RtK in 2 months (very doable, 35 per day) and used an SRS program to keep up my reviews with around 95% retention. This took 1 hour a day, then a scattering of 10 minute sessions (usually like 4-5) throughout the day where I'd do SRS reps. Every time I tested a Kanji, I had to write it in the correct stroke order or failed it.
RtK helped me meet those Kanji I didn't already know. It strongly reinforced the ones I did know. Now when I encounter a new character, I know I've seen it before, I can associate it with something in my mind, and it becomes much easier to remember the reading for it. As time goes on, the stories and English keywords just kinda fall away - they are indeed, useless. Heisig says as much himself in his book. What I'm left with is a strong impression of the Kanji in my mind that can then be (much more easily) built upon.
This also helped make new vocabulary acquisition so much easier - as when I encountered a new Kanji in vocabulary words, I didn't have to memorize that Kanji - just its reading (as I could already recognize it).
The system's real beauty is in teaching you how to break apart the Kanji into manageable parts - then rebuilding them when you want to write them. Before, when I looked at a completely new Kanji, I would just see a mess of lines. Maaaaaybe I would recognize some other Kanji or radical I'd studied before. Now, every time I encounter a new Kanji outside the 2100 or so in RtK, my mind systematically deconstructs it into its parts. This makes it so much easier to recognize the next time I see it.
This for only 2 months of study (with the caveat of continued reps on my SRS, for practice...). I think it was pretty worth it.
One more thing about the stories - they really do just sorta disappear. If you keep up your SRS'ing, you will be writing the Kanji so many times that there will just start to be a "right" feel to writing them, without need to call up the story. They kinda just fly out of your hand at that point, without much thinking at all.
I'm about 3 months off the end of RtK - my studying post-RtK is SO much better than before. Much less frustration, as now I actually feel like I at least have a little knowledge of most things I encounter - whereas before, I'd be staring at a newspaper article with at least 50% (the unknown kanji) complete non-comprehension...which was pretty rough. heh... and for most simple things (1 kanji words and many compounds) I can, at the very least, guess the definition if I don't have a dictionary handy. Then throw in some context and can get an incredibly good idea of it.
RtK is just like shaking hands with all the Kanji. Then your reading practice is where you really make friends with them.
Much earlier in this post, something interesting was mentioned in that there aren't many highly literate people that recommend RtK. I just think that most people that study Japanese aren't so aware of the system - or more likely, encountered it AFTER finding other methods. Eh, I don't have much of an answer for that. Maybe all of the benefits I've experienced are really short term and will just disappear soon.
Also likely is that I do study pretty hard, live in Japan, and get to use this stuff every day - I probably would've been doing pretty well without Heisig. But anyways, I do feel the time spent on it was well, well worth it.
One last thing to end this rather wandering post - I don't really like the criticisms of the story/keywords system. I just think this deserves special mention - like I've said earlier, the stories/keywords are useless (admitted by the author). They're only there to help serve as a mnemonic device that helps you recognize the character. For convenience, the keywords were chosen to correspond to at least one of the Kanji's meanings - but this is only just to provide a little extra help and to keep you from really confusing yourself a lot. Just use them to get the characters into your head, the writing down, then discard (replace, really) them as you learn their real Japanese meanings/readings.