People who tell you the ending ruins the show are people who care more about having sidequests resolved and a clean tidy wrap-up of the show than enjoying the journey. If you're a control freak who can't bear unanswered questions, definitely don't watch Lost. If you like a progressively complex and dynamic storyline with great characters, watch Lost.
The first season is definitely probably the best, because they only thought the show would get one season and the creators really did their best to make it something they would be proud of, even if it was a commercial failure. There are some moments in the 2nd or 3rd season (can't remember which) with certain love arcs that I find boring and annoying, but as the show progresses, things just get crazier and crazier and more exciting. If you watch a random episode from the 1st season and then compared it to a random one from season 5, it's almost hard to believe it's the same show. So, I would see this as motivation to stick around. What Lost does right is developing intriguing lore and feeding you enough information to keep you invested, but not too much to make it seem obvious what is going on.
There are some occasional cheesy moments and the writing does start to have some faults in the later seasons, but I easily managed to forgivee those things because of how fun and intense of a ride it is.