I picked up Snell a while ago and had to endure some horrific starts but he has settled in and won 5 in a row with some dominating performances. He has the skill set of a solid fantasy #3 or even #2: nice K/9, decent-to-good control, durability, NL in good home pitchers' park, and decent-to-good run support and relief support.
He's a rookie so you can expect some bumps in the road, but if you are looking for some cheap quality arms with upside on the FA list, then you should grab him if he's still there.
Ollie Perez is for those with strong stomachs only. There's no doubt he has the skills to be a #1 fantasy starter -- see his 2004 stats -- but he has been just atrocious since then (while still showing flashes of his former self) and just can't find the K zone. The upside is still there but the risk is HUMONGOUS.
Verlander and Hamels are also the real deals although they are probably long gone in all but maybe a 5-team league.
I was pretty bullish on Juan Cruz as well, but he has gone on the DL. Keep an eye on him. He too has endured some horrific starts but his underlying skill set is strong. I would also add Claudio Vargas to the list of bargain SPs with some upside.
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