They just fixed this problem. OmniFocus just updated the program a few days ago to version 1.1. Now, while the data is loading, the button to add to the inbox is active within 1-2 seconds.
Also, there is a lot of hints on their website forums on how to compact the data file so it will load much, much faster.
What about Appigo's ToDo??=] It's really great, it uses toodledo or RTMI gave up on Omnifocus today and decided to switch to Toodledo, only 3 second start up time with 150 tasks! Thankfully I never purchased the desktop version of Omnifocus for the hefty $80 price tag. (been running off of their beta releases) Omnifocus had gotten to the point where it was taking 16 seconds to start up on my iPhone even after a fresh restore to 2.1...not very useful when you want to quickly jot something down.