I love Nick Johnson as a player. A career .395 OBP is sweet, especially with a career .272 batting average. He also plays a very reliable-to-good defense at first base.
However, as others have said, he just can't stay on the field and generally isn't worth the gamble. In OBP leagues though (such as the one I play in), I think he might be worth a shot. Not saying he'd be someone I'd count on to start, but I'd definitely look at him as a backup and cross my fingers that he can replicate 2006 (500 AB, .290/.428/.520 with 23 HR, 100 R, 10 SB, 77 RBI).