I have a bunch of Garmin units. 200, 500, 510, Tactix, and a 910.
Out of those...I like the 510 the best. I like the bluetooth capability. It automatically uploads my rides onto my phone then Garmin Connect/Strava. The larger screen is nice when I'm trying to follow a downloaded map. I found this quite useful when riding some new trails in Vegas this year. The 510 does not do real time navigation like the 810...but it does navigate if you download an existing route. I also have a "Sunset" field on the 510. I often ride in the late afternoon and its nice to know how much daylight I have left.
If you're looking for a bare bones unit the 200 is fairly cheap and gets the job done nicely. It has no Ant+, so you can't use a cadence sensor or heart rate strap...for that you'll need a 500 and up.
I don't like the idea of using my phone as a GPS since the GPS will drain the battery at a faster rate...and if the phone signals are weak...the battery will drain even faster. With these data hungry phones of today...the battery use can be very unpredictable.