Aside from his business practices, I was first aware of Sinclair through the ZX81. I think he got the idea of the striking product design before Apple. The ZX80/81 was clearly a product for the future. I think the problem was there wasn't much great use for it beyond a novelty. I borrowed one as a kid and learnt to program on it. I'm not sure it had commercial games, so it wasn't as appealing as the Atari consoles for children. Also, it wasnt serious enough for business, maybe because of not having COBOL(?) or persistent storage.
I guess the Spectrum was the breakthrough because had enjoyable games. Still, instead of this I was able to use an Apple II. I couldn't believe you had to actuallly type out they keywords in BASIC!
Again, he seemed ahead of his time with his ideas of personal electric transport which is only just now becoming a reality. Its clearly a thing of the future, but local authorities haven't worked out how!