Bryan LaHair turned 29 years old in November and he's set to take over 1st base for the Cubs in 2012. LaHair was drafted in the 39th round of the 2002 Draft. Until a few years ago teams could do what they called draft and follow which meant they could draft a player, not sign him, but they retained his rights up until the next draft. The Mariners did just that with LaHair. They signed him in May of 2003. Five years later he made his big league debut and he hit .250/.315/.346. That was a .292 wOBA and a 77 wRC+.
In 2009 he was sent back to AAA and that's when he began to hit the ball very well. He hit .289/.354/.530 (.375 wOBA, 124 wRC+). He became a minor league free agent and the Cubs picked him up. He was even better then. He hit .308/.385/.557 (.402 wOBA, 136 wRC+) and followed that up last year by hitting .331/.405/.664 (.443 wOBA, 155 wRC+).
He set an I-Cubs record by passing Joe Hicks as he hit 38 of them. He led the PCL in wOBA, wRC+, OPS, OPS+, SLG, and HR. Mat Gamel finished in second with 28 home runs. LaHair has played in AAA for all or parts of 6 seasons and has nearly 3000 plate appearances there so the numbers are inflated, but that's a damn fine season.
After being called up last season he had 69 plate appearances and hit .288/.377/.508 (.381 wOBA, 138 wRC+). He won't hit that well over a full season, but based on his previous few seasons in AAA we should expect he can hit well enough to play at the big league level. Below are various 2012 projections for LaHair.
I'm going to guess that Jeff Baker gets a decent amount of playing time at 1st base against righties. Baker didn't have a very good season last year. From 2008 through 2010 Baker had wRC+'s of 96, 99 and 96. Baker got too many plate appearances last year vs right handed pitchers. I'd be surprised if we see that again. Baker's projections are shown below.
At some point next season the hope is that Anthony Rizzo will take over at 1st and be there for a long time to come. Rizzo had a disappointing MLB debut last year, but was dominating after returning to AAA. He wasn't too far behind LaHair among the league leaders in most offensive categories. Unfortunately I don't have access to his splits at AAA, but my guess is he started hitting lefties better than he previous had. Maybe not. It's entirely possible he did almost all the damage against righties.
If there's a concern for Rizzo it's that he hasn't been particularly good against lefties. Through 2010 he had hit about .300/.375/.510 against righties, but only .250/.310/.380 against lefties. That was only 339 PA against lefties so it's a small sample, but whatever damage Rizzo had done through 2010 it had primarily been against righties. He'd been an easy out otherwise. He was an easy out against both lefties and righties at the big league level.
Rizzo's projections are below.
If we estimate that LaHair gets 350 PA, Baker another 150 PA and Rizzo the final 150 we can get a WAR projection for the position. This gives us a Batting WAR of .7 for LaHair, -.1 for Baker and 0.0 for Rizzo. If we assumed Baker primarily plays against lefties and LaHair against righties then this would increase it to about 1 WAR for the 1st base.