I actually have one of the low end phones. It's the Virgin Mobile Oystr. Kycera makes it, not sure what the real model number is. You can do a lot already with the WAP browser and SMS. I can SMS my Google calendar and it immediately returns the day's schedule. I can also do it for at least 1 day looking forward. I can get to my Gmail account on the web browser too.
Almost anything that can be done on an IPhone can be done on just about any other mobile phone. It's not going to be as convenient, and certainly won't be as pretty to look at. But if you need access to data on the web, any WAP enabled phone can handle it.
Low end phones generally don't sync with a computer. However, if you sync your desktop applications to the web, all that data will be accessible via the WAP browser. All the major web mail options have a way to sync address books to your local PC. It's simple to route all your email through a web mail service like Gmail. Not only does it act as an automatic back up, it provides remote accessibility via the phone.
As a primarily "read" device, the low end phones are quite functional if you put some thought into how you can sync data to the web to make it accessible. As a "write" device, a tiny 12 key number pad is less than desirable. However, the never ending march of more features for less in mobile phones is solving that problem. Virgin Mobile has a new phone that provides a QWERTY keyboard, color screen, 1.3 MB camera and a bunch more. The retail price is $99.
In my view, web based applications are best because they will work the same from both the phone and a PC, and the web app will function as the sync point for the anything entered via phone or anything entered via PC. Also, relying on web based apps helps ensure that you don't get stuck with one wireless carrier just because you are hooked on a proprietary application available only in their phones. 12 months from now, the new Virgin smart phone will be on sale for $59 and within reach of just about anybody that wants or needs PIM type features in a wireless device.
Disclaimer: Contrary to the impression provided in this answer, I don't work for either Google or Virgin Mobile. I work for Techdirt and thus am not elgible for the payout.