I have been using a microsubscription model for an MMO strategy game for iPhone/Android that I helped develop. (http://spaceuncharted.com) We charge $1/month or $10/year for access to the full game. Here are some things we've learned:
-- Apple only allows auto-renewing subscriptions for magazines, newspapers and other periodicals. So, for us, users have to re-subscribe every month, which has lead to very poor retention rates. On Android, we use auto-renewing Paypal subscriptions, and have had more success.
-- Having to manage separate billing systems for iPhone and Android adds a lot of development time and headaches. The Apple billing system is particularly time consuming because the errors you get if something isn't configured just right are not helpful.
-- Subscribers can have very high expectations for a service that is costs $1/month. When they're paying, they (rightly) demand a high quality product. We've been very liberal with giving out refunds or credits when bugs have arisen or we've had server downtime.