Here's what I have so far:
C: Mike Matheny, Grade: A-
Matheny is a 3-time Gold Glove award winner who now leads all NL catchers in RBI's with 42. What more can you ask for? Great addition in the off-season by Sabean for the exception of that .240 batting average.
1B: J.T. Snow, Grade: D
All the Giants are basically getting from this guy is a former 4-time Gold Glove award winner who has been painful to watch at the plate this season. At least Snow provided a hot and cold bat that could deliver in certain situations last season, but this season, things have gone south. He is down in all categories (runs, home runs, runs batted in, on-base percentage, slugging percentage, and batting average) even with the extra at-bats.
Backup 1B: Lance Niekro, Grade: B
Niekro is a very young and talented batter who has a ton of potential. He should most definitely be the starting 1B in place of Snow. Niekro is hitting .292 in his first season with 10 HR and 34 RBI in only 168 at-bats. Given the full-time 500+ AB's, he could very well be around the 25+ HR and 90+ RBI mark.
2B: Ray Durham, Grade: C
Considering how he was projected to be a top 10 2B in terms of production this season and how he isn't even producing top 15 numbers at such a shallow position, Durham has been somewhat of a bust so far. Staying healthy is always a concern with him, and even though he's been able to stay fairly healthy so far this season, Durham has fallen well short of the potential 100 run, 20 HR, 70 RBI, and 15 SB he is capable of.
SS: Omar Vizquel, Grade: A-
At the age of 38, Vizquel has done all that has been asked of him so far this season while exceeding the expectations of most people. He's raised his batting average above .300, is swiping bags, scoring runs, keeping the strikeout total low, and drawing walks. In addition, he's been great in the infield with only 3 errors and has turned 46 double plays thus far.
3B: Pedro Feliz, Grade: C+
Feliz is WAY too streaky to be the 2nd option in the Giants' offense. At times he can carry the squad on his shoulders, but then again, he can't even hit a foul ball at other times. He can easily produce the 75 runs, 20 HR and 90 RBI, but not consistently enough for an offense that lacks that very consistency.
Backup 3B: Edgardo Alfonzo: D
Alfonzo is also one of these very streaky hitters, but unlike Feliz, he can't even put up the 70 runs, 15 HR, and 80 RBI. And now he has become injury-prone, great. It's all downhill for him from here.