I've seen the first four in one of my Friendly Local Games Store (is that what FLGS stands for ?), perused them quickly (easy, they're short), and decided I needed none.
Assassin ? Any class may be an assassin. The ninja-esque stealth killer would be a rogue. That latter multiclass in shadowdancer, for example. I've no problems with the assassin PrC using spells. I see them as a sort of death-sect or demon-cultists, learning arcane ways of performing their "art and duty". Somewhat similar to fantasy hashishins (who used drugs to bolster -- and fanatize -- themselves) or thugs.
Gladiator ? Bah, rogue/fighter.
Noble ? I already made, and played, my buffed-up-for-PC-play version of the aristocrat. Fundemental design difference.
Beside, between the PH, the DMG, and OA, I felt I have enough base classes.
For other Mongoose products: I have Seas of Blood, and I've seen some Quint. books. I'm happy with SoB. The material in the Quint books, from my glimpses, is not really of inferior quality to some WotC books. I would not allow everything in it; however, I would neither introduce forsakers, incantatrix or fang of Lolth. Between rule-compatibility and background-compatibility, I've plenty of reasons to refuse stuff from everywhere.
By the way, I don't object or regret it. It's normal. Remember the motto "options, not restrictions" ? It means it's the DM wh set the restrictions on what goes in his world, not the publisher.
On the other hand, there are also things that I liked in what I saw of the Quint. books, like the rules for apprentices in Q. Wizards, or the final chapter on building and managing one's fortress/tower/whathaveyou.