Hopefully Mozilla will make Thunderbird available with the Lightning plug-in. That is the best e-mail client I have ever used.
However with a mail client you can look at already downloaded messages if your internet connection is down or when none are available wherever you are at.
Not sure if the iPad version of Gmail allows it, but the full web interface version accessible through desktop browsers allows you to now download a cache of all existing messages (sent, received, drafts, etc.) to your system if you choose to, and any time you access Gmail, it'll update it and download the latest messages. I think Gmail even calls it "Offline Mode" or something, and it can be turned on and off via settings. So, uh, Gmail now fully offers this as well...
You can also compose messages and have them ready to send once your connection is back online.
This is available now with Gmail also... (well, maybe not iPad Gmail, haven't seen its full set of features. Will test it out today).
Personally I have never been able to stand browser based email.
I'm in an semi-opposite situation: while traditionally I've not been a larger fan of email web interfaces, I actually like Gmail, partly because using search through the web interface utilizes Google's own searching technology, so finding a message occurs really, really quickly. Plus, while Gmail doesn't have traditional folders, you can assign messages to as many labels as you would like AND, unlike a lot of email client search algorithms, doing a search for a single message "located" in multiple areas (or, in Gmail's case, tagged for multiple labels) doesn't result in several results returning for the same message.
When it comes to email clients, pretty much the only client I prefer is Outlook (PC). Then again, I do a lot of Exchange support, and pretty much nothing else even compares to Outlook...