The only knockoffs that I have ever found to come close in quality to LEGO products were the old TY-CO bricks and BTR (Built To Rule) bricks. All others seem to have lower quality plastic (brittle, soft, break easily), poor mold designs (stress points that break easily & low quality figures/animals), poor connectability/playability, and highly inconsistent colors. These are the main reasons I stay away from other brands. more simply put, and to quote others, inferior product.
That being said, the old TY-CO bricks were much better quality plastic, with almost identical colors to that of LEGO. They mainly just came in basic bricks with a few slopes and wheel assemblies, akin to the older metal axle LEGO variants. These TY-CO bricks came from the late 80's to early 90's I believe. Like LEGO from that time, the color options were very limited. I do not come across these very often, so I do not know if it would make sense to pursue them instead of LEGO product. I have no idea if there were ever any lawsuits or patent infringements. That just isn't something I care about.
The BTR bricks were produced by Hasbro around 2000 I believe. If memory serves me, I believe they had GI Joe themes. Feel free to research these if you want to correct me. As with TY-CO, I felt that their plastic was of good quality, and that SOME, but not all, of their colors matched the corresponding LEGO shades well. Again, I have no idea if there were ever any lawsuits or patent infringements. I'm not into legal stuff.
On occasion, I find random knockoff parts that are "good enough" for me to use in builds. Often times, I use blocks from other brands as structural components in areas that will not be seen. It allows me to save the better bricks for the seen areas. Anytime I do get these knockoff bricks, it is from a garage sale, thrift shop, or similar place. I do not by any other brands new. Anyway, that's my $0.02.
