Sure everything is supposed to be an app these days, but Kevin Purdy thinks Gmail on a mobile Web browser may be your best bet.
If you want to be notified immediately and repeatedly when you have new mail messages, then by all means - access your email through your Android Gmail app, your iPhone's Mail system, or even the BlackBerry add-on. They're generally good apps, they get the job done, and you always know just how far behind you are on your Gmail or Google Apps inbox.
But with the recent roll-out of some core new features, the version of Gmail you see in your smartphone or tablet browser is often the version that's more convenient and powerful than the dedicated app. This is definitely the case on iPhones, iPads, WebOS devices, and other portable devices not made by Google that have a solid, modern browser (usually based on WebKit). On most BlackBerry devices, the browser is pretty rough, so the official Google app might be worth sticking to. On Android, the Gmail app is as good as it gets - but, actually, you might still favor the mobile version, or at least want to keep it handy.
Google is a strident proponent of developing for the Web using cross-platform standards, and it shows in their mobile Gmail Web-based app. Here's how it holds its own, and often excels, compared to Gmail apps:
Multiple accounts? Check
Added just last week, Gmail's mobile site allows for easy switching between accounts by clicking a button at the bottom of your inbox and flipping over. It's nearly a tie with Gmail's Android app, which requires a tap on the account name in the upper-right corner for the switch.
Offline access and storage? Check and check
The problem with the Gmail Webapp, or any Webapp really, is that it can fall flat on its face when your Web connection goes away, or even just gets dodgy. Google's Web-based mobile Gmail lets you read recent messages while offline, as well as process your mail and compose messages for sending when you're back in Web-friendly territory.
Better search nearly everywhere
Until very recently, iPhone and iPad users who wanted to dig through their Gmail to find an address, phone number, or the like were plain out of luck. Even still, search on the Gmail mobile Web site can often be faster than asking your phone to search its own database, then reach out and look further on a remote server. Plus, you can use all the advanced search syntax you can remember to find exactly the message you're looking for.
Auto-responders and custom signatures
If you're headed out on the road or into a longer vacation, you can turn on an automatic "vacation responder" from mobile Gmail on the Web. Head back to the Menu (the arrow in the upper-left corner of the inbox), then look for the gear-like icon to set preferences. From here, you can write and activate an automatic response, as well as set a custom signature that lets people know you're writing from a small screen, if you dig that sort of thing.
Lots of little mobile things: Separate account shortcuts, storage space, better text selection, nicer looks
From Gmail's mobile Web interface, you can create a separate home screen shortcut for each of your accounts, whereas Android, iPhone, and most smartphone email apps don't allow that. And, depending on what version of Android you're running, you may find that the smaller and separate storage used by Web-based Gmail frees up some of your limited app space, and that Gmail in the browser is much easier to select and copy text from than the dedicated app.
And on every platform, you might just get addicted to the clever interface touches Google puts into their mobile Webapp: a floating action bar that prevents unnecessary scrolling, easy access to other Google apps from the shortcuts hidden above the top menu, and, on most tablets, a two- or three-pane interface that makes email triage sessions a bit easier.
Still needs work: attachments, more offline storage
It feels like a spoiled kind of nit-picking, but it would be nice if Gmail on the mobile Web allowed for adding attachments to new messages, even if it was limited to just pictures for now. And while Gmail does make use of some modern HTML5 storage powers, it would be better if, like its offline Chrome counterpart, the mobile Web Gmail gave users some idea of how far back their offline mail would go, and grabbed a bit more space if users wanted.
You also miss out on the inter-app communication that a dedicated app provides ("Email this" and "Share" links, in other words). But there's no law saying you have to swear loyalty to one form of Gmail or Google Apps Mail over another. Keep your app handy, but check out how nice, fast, and robust Gmail is rolling on the Web lately.