Guerrero is now on the decline in his career but he is still a force to be reckoned with. It was only last year, after all, that he was a top 5 pick in most leagues. He has now fallen to #13 but he will certainly provide you good value here. But be warned, he doesn’t offer you much upside, so if you want upside you may want to nab Sizemore instead.
Here's a little bit more about Vlad: What he will give you is an AVG over .300, 30+ HR and 10+ SB. Those are very solid numbers. What he won't offer you are numbers much better than that. He is on the downfall of his career at age 31 and it looks like is 40 HR days are over, but you never know.
One reason not to take him: He doesn't have a lot of upside. He hasn't hit over 40 HR since 2000 and he hasn't had more than 20 SB since 2002 when he had 40! He was at one point in his career a 40/40 threat but he never quite reached that mark and those days are long ago.
One reason to take him: Consistancy. You know what you are getting from him and he has a superb average to go along with his decent HR and SB numbers. There is always the chance he could nab you 20 bases as well.