OK, I'll jump in. I hesitated because I know nothing about this viewsonic you speak of.
I have an e310 and absolutly love it. (My first PPC) I previously had a Palm III ex. The e310 is easy to use, very powerful (esp with a good sized SD card) and small small small. (Didi I mention it's small) All these IT guys walk around my work with these huge iPaqs with sleaves on them attached to their belt and I pull mine out of my shirt pocket.
I have three complaints...
1) The buttons and directional pad on the face sometimes register twice when pressed just once. But it hasn't been a big deal.
2) (This isn't just about the e310. More about PPC's in general.) I was used to going to cnet.com and downloading tons of free stuff for palm. With PPC everyone wants $$$. Not that big of a deal really. Just could get expensive.
3) I wish it was Wifi ready. I just wasn't willing to spend the $150+ extra to go up to the e740, esp since my work was paying for it.
Over all, it does what I need it to do. Keeps me organized by being able to cary my contacts around. Keeps me on time by reminding me of meetings etc. And keeps me occupied by being able to download and design themes, play games and listen to MP3's.
Good luck! 8)