I've been using my Motorola Droid X as a mobile Wi-Fi hotspot. The extra $20/month service fee is worth it if you'll be traveling and if you need to connect up to 4 devices to the Verizon Wireless 3G network.
Overall, the 3G Mobile Wi-Fi hotspot feature works fairly well, as long as you have a strong signal to the Verizon network. I've been relatively happy. occasionally, I've found the service to stop, even when I have a strong signal, so I'm not entirely sure why that's been happening. I'll have to do a few more tests to determine how frequently that's happening.
Plus, I'm also finding that the connection speed may slow down. When I restart the 3G mobile hotspot feature, then the connection speed resets back to normal.
The 3G mobile hotspot feature will drain your battery relatively quickly, so be prepared with an extended battery or an external battery pack. I'm using a Tekkeon battery pack that can charge my Droid X or my Apple iPad, so I've got plenty of battery power.
I've also been using the mobile hotspot feature to connect 2 devices when I've been traveling: my Lenovo ThinkPad X200 tablet PC and my Apple iPad.
Would I recommend this feature? It certainly beats the alternative: you'll need to purchase a MiFi and pay much more for 3G service. You can also enable and disable this mobile hotspot feature based on your usage each month, so it's a flexible "option" for those of you who are using a smartphone on Verizon.