In the hickory era, the Macgregors ranged in quality. The best of them were great- the OAs and the Populars were flanged irons that repelled the turf a bit and had a lower center of gravity. They were better for the lusher turf conditions in the US.
Here are the Macgregors that I've found over the last few years. These are examples of what you'd come across a lot in people's garages. The duralite mashie (3rd down) is the best club of the bunch. I'd describe them all as solid players, but nothing special.
These are the better ones. OA mid iron and mashie on top- notice the flanges. Macgregor intoduced flanged irons like these as early as 1918, which is hard to believe. This was even before Maxwell started making flanged irons in the UK. Also below, some examples of bakspin irons. Usually they were mashie niblicks- about 43-47 degrees loft.
Also, I would say that the shafts in the Macgregors tended to be a higher quality than other American clubs. And by the end of the hickory era, they were higher quality shafts than the UK clubs, just due to the hickory supply here and I guess, their investment in the manufacturing process.