Not really a power guy, but with a little bit of pop. He does strike out a lot, but he also has historically walked at a good clip...though this slipped last year, as did his OBP. Not a standout hitter, but usually somewhere around league average.
He is left-handed, and historically has had good performance against righties (.348 career wOBA vs. R, .296 career wOBA vs. L). I'd caution against thinking he has particularly strong splits, because we only have 1500 career PA to look at thus far, 312 of which are against lefties. You need about 1000 PA's vs. lefties to get a reasonably strong read on a hitter's left/right splits (per The Book). Therefore, my read on him is as a roughly league-average left-handed hitter, with typical splits--meaning above-average performance against righties.
ZiPS and Oliver peg him at -2 RAA/162 games, which is more or less league average. I'll go with that, with the caveat that better leveraging of Lewis against righties only could increase his value to some degree.
Lewis is a very interesting case. He is by all accounts a fast runner, and has stolen 51 bases on the major league level (including 21/7 in 2008) and has hit 21 career triples in 1500 PA's! With that kind of speed, playing in a corner slot, you'd expect plus fielding.
Instead, his numbers look pretty average. His 3-year UZR/162 average is dead on league average. DRS is +4 runs per season. Not terrible, but not reflecting his apparent speed. The Fan Scouting Report might have the answer: it reports that he does indeed have plus plus speed. But he also is reported to have terrible instincts, bad hands, and an absolutely abysmal arm. His speed keeps him from being a liability, but he is no big help out there. Overall, the Fans rate him as a -4 run fielder per season.
A weighted average has him as a dead-on average fielder in left field. I'll go with that.
Position: -7.5 runs per season for playing left field, a relatively poor collection of fielders.
Replacement: +20 runs per season between average and replacement level.
Total Value: -2 + 0 - 7.5 + 20 = +10.5 runs per season.
If we figure 300 PA's this season, that would put him at roughly 5 runs above replacement next year, or about a 0.5 WAR (worth $2-3 million).
The Reds are paying only $900k, which makes this a nice deal for the Reds. They get a league-average hitter with good speed, and someone who might make a good complement with Gomes. He also will be under team control for a few more years, should he perform well. You have to wonder if some coaching could improve his fielding, at least as far as routes to the ball and such are concerned (the "instincts" part of the fan scouting report). Of course, I'd expect that his past clubs thought the same thing.