I still have my 60csx but don't use it often. It is still highly respected for its SiRFStar chipset and unique quad helix antenna however. I did some accuracy tests comparing it with some newer devices a couple years ago and the 60csx holds up pretty well
http://gpstracklog.com/2013/04/garmin-glo-review.htmlThe screen quality is a real drawback IMO, compared to today's standards. Very low resolution and only 256 color. It is highly visible without the backlight however.
Older devices like this are limited by the number of map segments they can access. These are the small tiles that make up a map. There are no standard sizes for segments - they could cover a whole state or just one town - it's up to the map maker. But the 60csx can only access about 2000 segments. That will prevent you from loading the whole US certainly. I don't use OSM so I can't be specific.
You should use Mapsource, Garmin's older (discontinued) software with the 60csx. There's a tutorial on this site. Download Mapsource here
http://www8.garmin.com/support/download_details.jsp?id=209Then you will need to use GMTK to prepare your computer before installing (unless you already have Garmin maps on your computer).
http://www.javawa.nl/gmtk_en.htmlThis explains how to install maps.
https://www.gpsfiledepot.com/tutorials/how-to-load-maps-on-my-garmin-gps-unit/#mapsourcePay special attention to step #8. Your 60csx only recognizes one map file - gmapsupp.img. Therefore, when installing multiple maps, they must all be encapsulated in a single file. This is done by repeating steps 2-7 for each map you want to install on the GPS.
Look at the Maps tab on the left in Mapsource. It will show the total number of map segments as well as the size. Make sure that you have not selected more than 2000 (IIRC the actual number is 2025?) segments.
Since the segment size is not consistent between maps, the only way to answer questions like "how many maps can be installed?" is to actually go through this process.
Newer Garmin models are better, but not as much better as you might expect. They can access between 3000 to 4000 map segments, depending on the model. Another big plus to the newer models is that they can use raster imagery - such as aerial photos and scanned paper maps. The 60csx can't do this. Garmin sells this kind of imagery under the "Birdseye" name, and you get unlimited downloads for a year for $30. Some current Garmin models include a one year free subscription.
You really should go to a store where you can see and hold the different models, you really can't appreciate that online. REI and the big sporting goods stores should have them.
The other thing you must consider today is… do you really need a dedicated GPS at all? After owning over a dozen and spending thousands of dollars over the years, I think I have bought my last one. I still like my Montana 600 although it's almost 5 years old. BUt I don't see anything compelling in the new devices.
I really like my iPhone 6s Plus in an Otterbox with a few different apps. I make my own maps, and these work well on the big screen. But I don't do any "serious" hiking, just a few hours here and there.
Have fun with th 60csx, it's a real classic. There has always been something about the look and feel of it that appealed to me. Feels solid and professional. The map limitations, awkward pushbutton interface and low resolution screen make it more of a nostalgia item than a tool that I regularly use though.