Setup wasn't particularly difficult, either. The Dialer app in Android can default to Skype for handling calls, so once you've sent everyone your new number and configured Skype, you won't really notice anything different in how you use your phone on a daily basis.
The T-Mobile/Walmart plan's 5GB 4G-data cap turned out to be more than enough for me--after a week's worth of calling through Skype, streaming videos via Netflix during my commute, and streaming music over Pandora and Slacker instead of using my MP3 player, I used up about 600MB --a little over one-tenth of my 4G-data allocation.
Smartphone power users, take note: Since the Samsung Exhibit II 4G is an HSPA+ 14.4 smartphone, it can't take advantage of the full 4G speeds of T-Mobile's HSPA+ 42 network, which some of the carrier's higher-end phones can use. On my speed tests, it reached 4.5 megabits per second downstream and 1.4 mbps upstream, with 52ms latency, on the high end; at the low end, its results were a relatively weak 863 kilobits per second downstream and 233 kbps upstream, with 586ms latency. Those aren't phenomenal numbers for a 4G network, but they aren't awful, either.
Is Switching Worthwhile?
Once I went back to my normal phone--a boring candy-bar Nokia also on T-Mobile prepaid service--I realized that the traditional cellular voice network isn't all roses, either. I encounter dropped calls, echoing, and other similar issues with my normal voice service, too. Ultimately, your choice depends on how much you're willing to pay for an always-on, comparatively reliable dedicated voice plan.
Personally, since I spend most of my time at home or in the office, both of which have Wi-Fi coverage, I wouldn't have to worry about missing or dropping calls there. When I'm out and about, the 4G network works well enough for me in daily use. And if I'm really worried about poor network reception, I can fall back on the 100 minutes of voice service included with the T-Mobile plan. So, for me, paying $39 per month for the T-Mobile/Walmart plan plus the Skype service is an absolute steal, considering that I'd be paying about $80 a month (before taxes and fees) for a comparable plan from T-Mobile itself, and even more than that for service from AT&T or Verizon. Over a two-year period, my savings would total about $960. I think that's worth the occasional inconvenience of a dropped call.