Santos has been almost exactly league after in his both his walk and strikeout rates over the past several years, with above-average hr-allowed rates in both Milwaukee and Pittsburgh. His ERA last season is scary, but his peripherals, FIP, PERA and BABIP (0.354) all indicate that he was fairly unlucky and actually produced at a level that was better than his '05 campaign. Even so, an ERA in the 4.5-5.1 range (average around 4.7-4.8) seems to be a good estimate of what we can expect from him. But he should be quite capable of producing at that level as long as his age doesn't catch up with him prematurely (he won't turn 31 until October).
The question is whether that is good enough for the #5 spot in the Reds' rotation. We certainly might hope for more, but given that half of all Reds pitchers who received 10 or more starts last year had ERAs over five, a 4.8 ERA pitcher is nothing to dismiss out of hand.
Competition for the Reds' rotation next year will be interesting. Harang is clearly the ace, and I think Arroyo and Lohse will both be respectable enough (4.2-4.5 ERAish). Milton is still a disaster (at least after the second time through the lineup), though the Reds seem committed to running him out there every fifth day. The #5 spot, however, is currently a complete free-for-all. By my count, there are at least six pitchers in the running: Elizardo Ramirez, Matt Belisle, Bobby Livingston, Santos, and most recently, Kirk Saarloos and Brian Meadows. And, on top of that, Homer Bailey might also be in the mix...and probably is better right now than any of those guys.
Once I get caught up on the transactions--and I'm devoting the weekend to doing so--I'm going to sit down and try to hammer out a recommendation from that list of pitchers. ... needless to say, there's no sure bet, though some are better than others.