I use an iPhone 3GS, and I thought that the Tree to Go would help me, as I previously had an HTCs620 phone, with my gedcom as a text file on it.
I find that the app isn't bad, but I use my FTM tree on my desktop PC as the master file. Every now and again, I upload a gedcom from that to Ancestry, then I have to download that to my iPhone. It would be easier to sync the tree from FTM on my PC, however it would probably need to only sync the non media data, or it would fill the phone if it synced all the media.
1 issue I have flagged back to Ancestry, is that here in the UK, the BMD registers are set up in quarters, so I regularly have peoples' births, marriages or deaths as, for example, Jan - Mar 1881, with a place that is represented by the Registration District. In FTM, this displays correctly, also on the Ancestry website it displays OK, but in the iPhone app it just appears as a string of zeroes. As I say, I've reported this issue to Ancestry, but updates haven't cured it yet.