I have an 810 which has had some issues especially with creating a route on a computer, uploading it to the Garmin, and then using it on a ride. Seems to get all wonky when you go off course and try to get back on regardless of whether or not I have reroute enabled. There were some additional bugs that have since been fixed with the current firmware updates. If I'm not following a premade route, it's fine. The map surprisingly doesn't have half the bike paths in my area which is annoying but there is a way to download free custom maps but I haven't bothered to try.

As to the 510, I bought a 510 for my wife last summer and surprisingly, it has been absolutely bug free. It has worked flawlessly and has all the essentials such as heart rate cadence, etc. As long as you don't need maps, it would be a good investment.

I've learned that with Garmin, more so than other companies, they appear to put out products without fully having all the bugs worked out. Great design and innovation but horrible software engineers, lol. It took over a year for my 810 to, for the most part, be problem free. Just look at the issues people have had with the 1000 since it came out. Give it about a year and I'm sure those will finally be resolved as well. Point being that the 510 has been out long enough that I think they've pretty much fixed any major issues.

Their mac/window software is still somewhat buggy. The new "look" is in my opinion awful but you still have the option of looking at the "classic" view. The biggest problem I've had with the computer software is, again, with creating routes for upload to the device. For example, when I'm trying to create route on a MacBook, it seems to freeze up before I'm able to complete it. Happens occasionally if I'm using a PC. It's also somewhat sporadic in recognizing that you've hooked your device to your computer so you can upload/download rides and update software.

All in all, you should be very happy with the 510. Hopefully you can get one at a great price too.