I'd like to say that "Missing Pieces" has been hit-or-miss, but that would imply that they're hitting with the same frequency that they're missing. The overriding theme through the first 3 mobisodes, at least, seems rather bland - seemingly self-satisfied Losties don't know just how badly things are about to go. It's dramatic irony played to cheap effect, mostly, though the latest mobisode has a bit more going for it than the others.
1: The Watch
Christian gives Jack the watch this his dad gave him as a wedding gift, though Christian's dad meant it as a sort of curse. Beyond being something of an ambiguous character-moment for Christian - is he sincere when he says he thinks it'll work out for Jack? - not much is happening here. Geoff objected to the attention paid to the watch when we already have an important watch on the island, but I'm fine with it. The show is certainly obsessed with time, counting, and numbers, so it's consistent, at least.
2: Hurley and Frogurt
Frogurt threatens to make a move on Libby if Hurley doesn't first. Aside from the irony, I can't tell why this is necessary at all.
3: King of the Castle
Jack and Ben play chess while chatting about their deal to set Jack free. This is the first really interesting mobisode. The dialogue is painful in letting us in on the main joke - there are references to blowing up the sub, which happens, and to Jack wanting to return, which happens - but the chess game provides some additional interest. It's not immediately obvious - a number of websites, I've noticed, have mistakenly said that Ben wins the game - but Jack puts Ben in check and Ben 'castles' in order to escape. The comment, then, is not on Jack's attempt to escape and Ben foiling it, but rather on Jack seemingly defeating Ben (in the season finale?) and Ben slipping through his fingers (which has yet to happen?). Finally - and this is something I learned through lostpedia - the castling move that Ben uses is actually illegal, since Ben's king has already been moved. I can't imagine that this would be a mistake, but it would seem to imply that, if we can take the symbolism at face-value, Ben will continue to manipulate the rules of the game.