I base my opinions on where they were drafted and their contribution:
Chung: B+/A- Solid player. Could be great or even elite over time.
Ron Brace: B. Lost rookie season. Has been pretty solid as a rotational guy this year.
Butler: D, as a second round pick, he should be at least a solid contributor. Stinks right now
Vollmer: B+, Had a great rookie season. Has struggled at times this year
Bradon Tate: B-, Great return guy, but very inconsistent WR
Tyrone McKenzie: F, Gone from the team. Not all his fault since he tore his ACL. With the Bucs now
Rich Ohrnberger C- A back up, but not seeing the field
Gary Bussey F, Gone
Jake Ingram D+, near perfect at long snapper last year, miserable this year. Cut from the team. Doesn't get an F because he was a fifth rounder
Myron Pryor: A Solid rotational guy for a sixth rounder. If a sixth rounder can make a consistent impact at all, I say he deserves an A
Edelman: A-, Lost in the shuffle this year. Incredible rookie season. If a seventh rounder makes any significant impact at all even if it is only for one year, he deserves at least an B+/A-. Most seventh rounders never see the field in any significant capacity.
McCourty: A+, Legitimare DROY candidate. Has played shutdown at times. Covered Megatron as good as anyone in the league.
Gronk: A, Great at all phases. Also another rookie of the year candidate.
Cunningham: B, Started slow because of injuries. Shows flashes of dominance, but not consistent. A position that it can take a year to hit your stride.
Spikes: B+ , Good ILB. Can get lost at times in coverage. Likes to hit. Solid against the run.
Taylor Price: F, not being activated. There is a glut at WR and could easily become a solid player in the future, but since he isn't dressing on game days and is a third round pick he deserves nothing higher than a failing grade.
Hernandez: A, Started out as a solid ROY candidate. Has fallen off recently, but he has been a difference maker and for a third round pick to make that kind of impact right away is big.
Mesko: A-, The best punter the Pats have had here in 7-8 years.
Ted Larson: F, A sixth round pick who is now on the Bucs. Granted he did start for them, but he isn't doing anything for the Pats.
Brandon Deadrick: A, The guy is a seventh round pick who is getting decent rotation in the d-line line up and even starting games. Got two sacks. Incredible pick for the seventh round.
I will forgo the other guys because they are gone.