One feature offered by the Connects, and that the Play:3 and Play:5 systems lack, is audio inputs. The Connect has a single-stereo RCA input, which can be used to connect an iPod dock or even a pre-amp, which would enable switching between among different sources. To connect the Sonos to your stereo system, you have the choice of either analog out or digital coax/optical.
The Sonos family of products is designed as a multiroom audio system, and connect to one another via a proprietary wireless mesh system. The advantage of that is that it's not limited by your home's Wi-Fi network. The drawback is that there needs to be at least one wired connection between your home network and a Sonos unit. (Think of it like a DECT cordless phone system: one base station needs to be plugged into the wall jack, while the others communicate with that one wirelessly.)
If you're looking to add the Sonos Sub to a system featuring the Connect, then unfortunately you're out of luck, as that device is only supported by the amplified Sonos components. However, adding a sub of your choice to your existing hi-fi is a better option anyway.
If you're an audiophile, then the idea of the Connect will be much more compelling than the Connect:Amp. The inclusion of the digital output enables users to connect their own Digital Analog Converter (DAC) for a potential sound quality upgrade. While the Logitech Squeezebox Touch saw very little benefit from an outboard DAC -- its onboard sound is that good -- there was room for improvement with the Connect.
Nevertheless, sound quality of the unit was very confident, but like the Connect:Amp a little lean in the bass. The more expensive Connect:Amp demonstrated a greater sparkle when paired with Bowers and Wilkins speakers than a combination of the Sonos Connect, Marantz receiver and B&Ws. Adding the Cambridge Audio DacMagic into the latter mix helped with the lean bass, but I still preferred the sound -- and all-in-one convenience -- of the Amp better.
In general, the software is quite easy to use, and the capability to make playlists on the fly and control multiple zones around the house on the fly is the Sonos system's greatest strength. If you can use a tablet, you can use the Sonos.
As I outlined in the Connect:Amp review, the interface does have some niggles, but as the Connect isn't amplified the touchy volume control doesn't matter as much. Since at home I use a mix of local lossless files and Spotify almost exclusively, I only wish that the Spotify integration was better. The Logitech Squeezebox's software is more powerful, offering most of the features of the desktop Spotify app and if you use this service often I'd suggest getting the Touch instead (while you still can).
The Connect is an excellent combination of hardware and software that offers a plug-and-play solution for existing stereo systems. I'd only suggest this product if you already have an existing Sonos setup or are looking to get one. If you just want to play network music on your main stereo and aren't interested in a whole-house system, then the Apple TV is the best way to go. It's a pity that Logitech has discontinued its Squeezebox range and replaced it with the UE Smart Radio as it was the only real contender to the Sonos Connect.