Not bad at all. That said, having a favorable H2H against a Grand Slam winner is the only noteworthy aspect of his otherwise lackluster career. And Del Potro will probably end up overturning that H2H anyway; although Ferrer does pose some matchup issues to JMDP, Juan is clearly the better tennis player by far and the H2H mostly an anomaly.
No no, you're quite incorrect. Both are quite similar in the regard of having mediocre careers. If you take away Del-Shitro USO, both failed to win masters, both only win MM events, both fail against top 3 and both are seen as tier 2 players. David is a better player obviously, with better H2H, better ranking and more consistent game. Del Shitro is just a ballbashing clown whose fans are still hiding behind his limp wrist as an excuse that he can't beat a 31 year old Noserer.