I think it is a weak feat, and I use a house ruled version of it.
BUT, I seriously doubt a single classed sor/wiz would survive through the first three to five levels of any game I ran without taking toughness in some form, which is why I don't like versions of toughness which nerf it for low level wiz's/sor's.
For a first level wizard, say 14 CON, toughness improves your h.p. from 6 to 9. That is HUGE. The benifit at 2nd level, from ~10 to ~13 is equally huge. Unless the DM goes soft and deliberately keeps a spell caster alive, very few characters are going to survive with less than 5-6 h.p. per character level if they have to face encounters of CR = thier level.
Slomoot: I have a similar homebrew feat called 'Hard to Kill' (it a little more helpful than the one you describe), in edition to the house ruled toughness.