[eluser]Chris Newton[/eluser]
If you think PHP is a good basis for web application development then there is no reason not to use CI. The great thing about CI is that it does not rely on a lot of strict conventions. If at some point you choose to eschew some of it's methods & functionality in favor of plain PHP you're not really locked into anything.
Personally, validation, encryption, active record, form helpers are what have me absolutely loving it. I just recently built a custom shopping cart in under 2 weeks, including a user login system with password retrieval, database operations for adding phone orders, with CURLed XML output to a fulfillment center, automated CSV export & import. I would have never been able to complete something that fast and that ambitious without CI... let alone something that I feel is relatively secured against vanilla hacking attempts. I think CI is a powerful tool in my PHP arsenal, and it makes me enjoy doing what I do a little bit more.
The only thing that I really wish CI had built in was support for many-to-many database relationships. Editing & displaying many-to-many database table relationships is still a pain, and relies on a lot of complicated SQL queries.
