Caldwell was 2006. Stallworth was 2007, and had lost his starting job by about midseason. Lloyd & Co. not being complete embarrassments to the sport of football doesn't mean that they were successes.
I liked Lloyd for the team, despite his limitations. I thought they should have kept him around and just added another piece. It was the team that decided he was not a success, not me.
From 2007-2015, the Patriots have either not a single successful WR FA signing, or they've had one, depending upon your take on Amendola*. That's just the math of it. I, personally, don't consider Amendola a success (Remember, he was supposed to be the successor to Welker), but he's not a complete dud, either. If there was a way to give a signing half credit, that's where I'd put him.
* To be fair, one could really call Branch a successful FA signing, rather than viewing him as a returnee.