I've never noticed any lagging in rockbox. However I've been using rockbox exclusively since around 2004 or so, and haven't really tried the of in any of the devices since then.
About IEM's - it's really a matter of personal taste. I've got a bunch of them, but the etymotic hf3's are what I use the most. I haven't compared them to other ones by etymotic - can't afford them. And yes, the build quality seems a bit shady, but I haven't had any problems with mine yet. I also have some Shure se115's, which also sound great, but are uncomfortable, although they're built like tanks.