I have a few friends who have done just the same thing, although some have chosen Google Play Books, and others Kindle. I'm among the last ones from among those for whom I know syncing annotations is essential, who's still clinging on to Marvin.
We quite narrowly avoided a major catastrophe in Marvin 3: Kris was ready to ship it with the option for exporting annotations in HTML
removed, as opposed to Marvin 2!

(And that's the trouble with Marvin 3: overall, it's a nice improvement over Marvin 2, but some extremely useful Marvin 2 features have disappeared, and haven't returned since: let me only mention the missing three-finger-swipe gesture in Marvin 3, or the lack of signaling the presence of an annotation when it's attached to a highlight – in Marvin 2, this was shown by the highlight being underlined with a dotted line.)
So I certainly don't wish to boast here, but if anyone finds exporting annotations from Marvin into HTML useful, the only reason we still have it in Marvin today is because I pleaded like crazy with Kris, in the final stages of the (limited) Marvin 3 beta-testing, to restore the feature. And Kris did, at the last minute, which goes to his credit.
I can tell you, rfog, that if Marvin 3 indeed lacked the option to export annotations into HTML as in Marvin 2 (and it's an extremely buggy and deeply unsatisfactory export operation, but at least it's there!), I'd probably have deserted Marvin a long time ago, too.
So what I do instead now, is that after I finish reading a book (I have 3 iPads and 1 iPhone), I email the HTML export of annotations from all 4 Marvins to my desktop machine, then I fire up the SeaMonkey Composer there, and I manually collate all 4 HTML annotations files into one. (Yeah, theoretically, Marvin should be able to merge all those files on its own, but that process has been known to be buggy, too; I want to play it safe.)
You see, much as I hate the lack of highlights and annotations syncing in Marvin, when it comes to "digital typography", the way e-books
look while we read them on the screen (and that is the
one thing we spend most of the time with while using e-books!), I still find Marvin the best of them all – better than MapleRead, Hyphen, or Moon+ Reader on Android. (I have yet to look at tiReader.) Those other apps are pretty fine as well (while sharing many deficiencies with Marvin), but typographically speaking, in terms of rendering, Marvin still has an edge over them, I think. I'm not sure how much longer this prerogative alone will be able to keep me attached to Marvin... because those other apps, unlike Marvin, seem to be getting more updates recently, so there's reasonable hope they will be improving faster (and removing their current deficiencies) compared to what looks like a pretty stagnant Marvin in the last couple of years (minus the short-lived Marvin 3 outburst), unfortunately.