Well we seem to start with which mini figure comes with it and then go from there
We HAD to get General Grievous' ship just for the general. It did come with a Jedi and a droid. The ship does not stay together super well. The ARC-170 my mom found at Walmart on sale for half off last January (I think it was a mistake) holds together well and has these cool wings that open and close but no one that DS was super excited to get. We also have the Bounty Hunter's Assault Gunship, it comes with a lot of guns but doesn't stay together very well. We also have a Tie Fighter that I bought on sale last year it holds up well but I don't see it on Lego.com anymore.
I really want the Millennium Falcon, it comes with lots of cool characters. I think we might have to talk to Santa about that one
For the most part I don't think any of them hold together very well, they have some cool features like sliding doors etc but because you often have heavy engines hanging from thin wings they just fall apart a lot.