Is a 3.5" screen Garmin GPS still usable?
Answer: Yes. I use one myself as a daily driver.
The very last 3.5" screen Garmin car-specific GPS ever released for North America was the nuvi 30 in 2012. The 30 was launched with hardly any fanfare, lived for a short time, disappeared, and we never saw another 3.5" car GPS after that from Garmin.
I own two nuvi 30 GPSes because I like them so much. If my main 30 ever busts, I have a backup.
The 30, just like any 3.5" Garmin GPS, works the same as widescreen models, with the exception all of them have the ABCDE keyboard layout and no QWERTY. But that's literally the only difference. The 3.5" is functionally identical to widescreen models otherwise.
What makes the 30 great are two things. First, just like the 50, it has that glorious daylight-readable screen that was the best generation of screens Garmin ever put on a car GPS. Second, the screen touch response is far superior to that of the 40 or 50 of the same generation. For whatever reason, 3.5" matte screens always had better touch response compared to wide 4.3" and 5.0" matte screen models, and the 30 is no different.
Text too small?
The map text is fine, but in the Favorites list the text is a bit "scrunched".
Because the 30 model has such good screen contrast like the 40 and 50, it's easily read during bright daylight driving. For night driving, obviously there are no problems with legibility there.
Again, on the Favorites list is where the smaller text gets a little annoying. But it's not a deal-breaker.
Map too small?
In Track Up or North Up map display mode, yes, too small. But in 3-D mode it's just fine.
The 3-D display option has big thick on-screen map arrows and increased text size that can be read easily on the 3.5" display.
Works?
Yes, the 30 works very well as does just about any Garmin 3.5" with updated firmware and maps, even if it's free OSM map data.
Good enough for 2019 and beyond?
Yes.
A Garmin nuvi 30 or any other 3.5" model for that matter is a means of car navigation only used by nerdy folk who appreciate simple, reliable GPSes.
You have to be nerdy to use one of these things. If you buy one, it will be old, it will need its firmware updated and will need new maps (usually handled by a free OSM map image).
I was willing to go through the process of doing all the updating to get a nice, reliable 3.5" GPS with a truly daylight-readable screen. The 30 doesn't do traffic nor Bluetooth nor Wi-Fi nor connect to a phone and never did. All it does it navigate. And it does a damned good job at it.