I've owned and used the 2015 Shield for about 8 years and had a positive experience overall. My primary use is media streaming through online streaming services and via Plex (local on a NAS and remote from some friends across the country). For this, it's been a smooth and snappy experience.
My 2015 Shield is definitely showing its age (no Dolby Vision support, which the 2019 version does have), but it's codec support otherwise is excellent, especially for lossless audio like DTS-HD or Dolby TrueHD.
I've streamed games from my computer a bit, but you should be aware that official GameStream support was ended about a month ago. There's alternatives (such as Moonlight) but I haven't tested them out yet.
That said, I'm of the opinion the Shield could use a refresh. The most recent 2019 version is still fine, but the processor isn't significantly more powerful that other high-end streaming devices. And it could really use AV1 codec decoding (which admittedly very few other devices support yet).