Interesting question. Lendl BECAME a great player and a champion. He should definitely be mentioned in the same league as Connors, McEnroe and Agassi as a player, his persona was different. Lendl came from Czechoslovakia as it was then, and must have had a different upbringing to the Americans, he wasn't brash, he let his tennis do the talking.
I was a Connors and Mac fan, so I didn't mind Lendl losing those early finals.
I always thought he was a very good player who couldn't win on the big occasions, I guess you could call him a 'choker' to a certain degree. Lendl overcame that somewhat though and DID start to win the big tourneys. 8 GS is not a bad haul. Mac of course choked himself in the RG final 84 for Lendl's first GS win.
The fact that Lendl never won Wimbledon is the biggest knock against him when you look back at his career. He made finals and semis but always found someone better than him on grass when it came to it. Lendl was a very consistent player overall, regular tournament winner on the tour, clear no.1 for 3 straight years. 8 US finals in a row is outstanding.
I rate Lendl highly myself. He worked hard and made the most of his ability. All time he's a player to be considered a great in his day.