1) Google infrastructure is probably more secure than any server you could set up quickly. 2) Google handles all those pesky issue with IP blacklists, Spam, etc that you'll be forced to deal with on a DIY server. Granted, there are solutions for each of these; Its your valuable time, in the end.
On the other hand, for a DIY mail-server: you truly don't gain much besides having an accessible dump of your mailbox(in the event that all the gmail-specific datacenters get nuked in a single day?)
ISPs can and probably wil sniff your plain text email traffic now/in near future due to external police state pressures.
I live in India, and sadly I envision this place turning into a police state by sheer incompetence on the part of the administration who pass half-baked net-nanny laws.
The only solution to email privacy: Hard-to-reverse Encryption of the text (typically GPG). Most of the crowd here know this well. But making email encryption transparent and easy-to-use would be a killer product for those customers that value privacy. FireGPG is trying , but Gmail code changes keep breaking stuff sometimes.(http://blog.getfiregpg.org/2009/11/05/gmail-issues-fixed/)
Redundancy/Backup is another issue altogether. You should probably keep backup IMAP dumps in a geographically distant location if you fear Gmail will just cease to exist one day(without any warning).