Although Ooma appears to be a fine company, the problem I tell people is that that is a proprietary system. That means that means that you cannot use it with other providers.
The Obi devices can interface with many other providers and with Googles voice service. While Google Voice seems to work fairly well, you get what you pay for. Being a free service, it has some issues. Still, free is great if it works for you.
With Obi devices you also can have multiple providers for inbound or outbound calls. This also allows you many other options which various providers may or may not offer. That includes multiple voicemail boxes, recordings, call directing, telemarketer blocking, follow me and multiple device simultaneous ring and time of day routing.
I have a free GoogleVoice number but also pay for other numbers for various reasons. For example, I have a special number which gets published for the domain names I own. That way the scam/spam calls which arrive through that number can be handled outside my regular call flows.
I also buy numbers to use for special events like publishing on Craigslist to test marketing a product announcement. The cost might be 40 cents to buy a number and 85 cents a month if I keep it.
I personally have multiple Obi devices (110, 200, 202 and a couple of 2182's). The latter is a nice system but a bit harder to set up all the options I want to use. However being able to put calls easily on hold, transfer calls and have multiple numbers show on the screen is very useful.