I just saw this last night! I am actually very happy that Ancestry.com pushed this app out as quickly as they did. It shows that they are not going to ignore a potential Windows RT market.
I agree that you can basically just look at people, and you may have to work to find the person you want to see. However, I think it has great potential and the only screen that looks cluttered is the home screen (the personal details screen I didn't find that bad at all). I did notice that it links you to IE10 via a link at the bottom right to send you to Ancestry.com for any actual work.
I am leaving it installed and waiting for some updates, which I hope will make it an app that is at least as good as the iPad version.
I think it would be amazing if they eventually were able to bring FTM into a Win 8 app (even a lite version). That way I wouldn't need a computer to run FTM on trips-- a Win RT tablet would do the job at a fraction of the price.