Okay, here's my experience at using the Nexus 7 as my only cell phone for the last 2 months. At work, I have WiFi access. At home, I have WiFi access, the problem was when I wasn't in one of those 2 places.
I attempted to solve this problem with FreedomPop. If traveling where there was strong 4G Sprint coverage, I could make and receive calls. In my experience, Sprint's 4G coverage wasn't in the places I need it. Plan B is the Verizon Jetpack. It wasn't free, but still cheaper than a cell phone. I've been using it for the last month, and it is much better for my area (North Atlanta). I'm on the 4GB plan, and share it with my wife. I'm not on the Jetpack often, and could possibly drop to the 2GB plan. I'll wait a few more months to find out.
I use Google Voice for voicemail and text messages, and Groove IP for my dialer. I have a bluetooth headset (Plantronics Voyager Legend). Here's the skinny:
Call quality: With strong 4G signal, or good WiFi signal, the person on the other end cannot tell I'm not on a regular cell phone. I often can, as the person on the other end's voice is slightly clipped. In my truck (which is noisy), the noise cancelling of the bluetooth headset makes it hard for the receiver to know I'm in my truck. I do wish the volume of the Plantronics was higher, however. I have to press the earpiece into my ear at highway speeds.
At low quality 4G or poor WiFi bandwidth, the receiver's voice cuts in and out. The worse the signal, the worse it gets. I installed a WiFi repeater in my home so there are few places where I have this problem while there. While on 4G, pulling over often solves the problem. While on WiFi, moving closer to the hotspot works well.
Interestingly, dropped calls are not much of an issue for me. Even if the caller drops out for a few seconds, moving for a better signal restores it. In my limited experience, I'll notice it before the caller does.
The downsides:
MMS messaging. Messages sent to multiple recipients or picture messages are not received. The sender doesn't even know I didn't get the message. Google Voice simply doesn't handle them (except for messages from Sprint phones).
Extra devices. I have to bring my tablet, my bluetooth headset (which I could live without in a pinch), and my Jetpack (which has a battery life of 3 hours). That is sometimes a pain, but I'm rarely without my tablet, and the other 2 devices are small. The gym is about the only time this is an issue. I don't mind not being able to answer the phone, but it is difficult to work out without my tunes.
Voice commands. There may be a way to do this, but I haven't figured it out yet. I can't tell my bluetooth device to call a number or answer a call. The bluetooth device says "No phone connected"
That's my somewhat long-winded explanation of how it has been going with the Google Nexus as my oversized smartphone.