I have owned and use the Tamron G1 & G2, also the Canon 100-400 mk I & mk II (with 1.4x III).
The G1 was good, the G2 is better. Of the 4 150-600 lenses available currently (Tamron G1 & G2, Sigma C & Sport) the reviewers list them like this:

Best built/weather sealed G2 & Sigma Sport

Best Image Quality G2 and Sigma C

Best bang for the buck Sigma C

Best if you are on a tight budget G1 - it was and is a good lens if I was still using one (my first really long lenses) I could get by, I might wish for an upgrade but I could get by just fine.

One reviewer also added that the fastest AF is the G2, followed by the Sigma Sport

If I am after maximum reach I use my G2. For maximum IQ I use my 100-400 mk II. I can get almost as much reach with my Mk II + 1.4x but the combo is f/8 so needs more light than the f/6.3 G2.

Here are 4 shots, the first with the Tamron G2 and the second with the Tamron G1 the third is with the Canon 100-400 Mk II and the 4th with the Mk II + 1.4x III just to show a comparison with a more expensive lens.