I wouldn't call Tex's contract worse than Giambi. He's been hurt once and is a superior defensive first baseman. And will still mash once healthy.
Just for fun, let's compare their first 5 years with the team
Giambi (7 year, $120 million deal - $17.15M/year)
Year - Age - GP - BA - HR - RBI - BB - SO
2002 - 31 - 155 - .314 - 41 - 122 - 109 - 112
2003 - 32 - 156 - ,250 - 41 - 107 - 129 - 140
2004 - 33 - 80 - .208 - 12 - 40 - 47 - 62
2005 - 34 - 139 - .271 - 32 - 87 - 108 - 109
2006 - 35 - 139 - .253 - 37 - 113 - 110 - 106
Teixeira (8 year, $180 million deal - $18M/year)
Year - Age - GP - BA - HR - RBI - BB - SO
2009 - 29 - 156 - .292 - 39 - 122 - 81 - 114
2010 - 30 - 158 - .256 - 33 - 108 - 93 - 122
2011 - 31 - 156 - .248 - 39 - 111 - 76 - 110
2012 - 32 - 123 - .251 - 24 - 84 - 54 - 83
2013 - 33 - 15 - .151 - 3 - 12 - 8 - 19
Hurt once? He played 123 games in 2012 and 15 last year. In 2010 and 2011, he hit .256 and .248, respectively. I have a very difficult time looking at Tex's numbers and seeing a better player. Maybe you could argue that simply replacing Giambi's production was good enough. But the problem I have is Tex started 2 years younger...and by the time Giambi got to the 5th year of his deal, people were calling for his head.
But I do agree, Teixeira definitely was a superior defensive first baseman...but after that wrist injury...I'm not so sure anymore. To me, he's got a lot to prove this season. Because personally, for the amount of praise he gets, he should be more than Giambi 2.0. It's also worth noting that Teixeira's BB/SO ratio is getting bigger and bigger.