Whether a oil is good or not truly depends on how well it performs in your engine. There are many Gp 3 based oils that perform incredibly well.
As for Shell's Rotella-T synthetic oils, they are group 3 based. This does mean that they're made from a severely hydrotreated mineral oil. However, they are classified as a XHVI (extremely high viscosity index) oil and therefore give performance nearly equal to PAO based group 4 products. At this level of refinement the additive package of the lubricant is normally what differentiates one product from another.
Generally speaking, the quality of a gp4 oil is more reliable than is the quality of a gp3 oil. This is because one oil comes from gas molecules and the other oil relies upon mineral crude to provide the base. Not all gp3 oils are equal and this is seen through OEM testing results with different brands of lubes. Why would Castrol's GP3 based 10W-30 pass a sequence test while Valvoline's gp3 based oil wouldn't? It's purely because of the purity of the refined feedstock.
So don't make your oil selection based solely on the basestock of the oil. Mobil-1 makes a very good product and it is gp4 based, but Mobil-1 doesn't always perform the best in every engine. Some instances will show Castrol's gp3 oils perform better than Mobil-1, especially with regard to wear metals in the heat affected areas such as upper cylinder bore regions.
And with regard to Shell Rotella-T 5W-40, in my opinion it is one of the best oils for the price that you can buy.