My first impression is that this is very promising. Features are very well chosen not to complicate things but still powerful enough to manage apartments. Comming from online softweare development myself I know how hard is to achieve that.
Anyway, here are my showstoppers:
1) Curryncy limited to $ - this must be very high on your list unless you target USA only and that would be a shame for such a small feature. Maybe not complete multilanguage featurelist, only curency label to change (don't know about payment gateways)
2) Second billing due - validation rules do not allow it to be se to 0. here in Croatia it is very common to take 30% advanced payment but remaining 70% is payed in cash on arrival!
3rd important feature I miss is notshow stopper but I need it badly:
3) Reporting - maybe basic reporting or at least full export of data so we can do it in excel. Sometimes I need it for some tax reports (visitors list) or I need it for management decissions (stats on bookings, accepted offers, etc.)
Anyway.. congrats on a very good start and wish you luck in your future business.