I use Lydia all the time...mainly as a pack mule because I'm into alchemy and I collect tons of ingredients over expeditions (usually 2-4 dungeons/quests before returning to my home in Whiterun). I also found her very useful at lower levels, to soak some damage from stronger enemies. Now, I just bring her along to carry stuff for me. Some of the path finding is annoying, but it really doesn't bother me. If she gets lost, she will catch up eventually, or when I fast travel or enter a new dungeon/cell which requires a load.
I really don't have any big complaints about companions. Sure, their AI can be dumb at times, but I just do my own thing and make sure I don't hit/kill her while doing battle. She's kinda endured herself to me at this point and I enjoy her pathfinding stupidity from time to time

. Of course...if you're playing an assassin / thief, then you are probably better off without a companion. While they are decent and following you while sneaking, they are a bit dumb when sneaking and battling stronger opponents.