I broadly agree with @KRyan's answer, but I really like the flavor of spring attack: it feels cool to be able to use, even when it's mechanically unsound. So, setting aside all of their correct(!) criticisms of the feat, I'm going to try to answer with the assumption that "I'm taking spring attack" is desirable on its own for whatever reason. So: let's just assume that all references to spring attack in this answer carry an implicit "to the extent that it's ever useful" assumption.
Also, note that enemy reach here mostly means "compared to your own reach"; it got ugly including that, though; "without reach" means "without reach that's larger than yours", basically.
Faster will always be better, especially with feats like spring attack. There is no point at which more speed stops being useful at least once in any campaign I've ever seen or heard about (granted, there may be only one time that having a speed over 9,000 was useful, but that one time was Really Cool™).
The benefits of speed will also highly depend on the areas in which combat takes place. Fighting on an infinite, featureless plane favors speed and spring attack far more than fighting in a divinely-enforced 10x10x10 cube.
That said: to my analysis, the utility of spring attack kicks in at a speed of 10 feet, increases quickly to a speed of about 30, then very slowly increases to whatever maximum speed you can manage to attain.
At a move speed of 10 feet, you can use spring attack to avoid the negative effects of any creature with a 5-foot aura that affects you during their turn and/or on the beginning of your turn, so long as the aura doesn't also trigger when you enter it. Granted, there aren't a whole lot of such creatures out there.
At a move speed of 10 feet, you can also avoid melee attacks by immobile creatures without reach, which is even better than having an arbitrarily high AC (again, though, the count of such creatures is quite rare, unless you're in a party that can apply similar conditions). In this one very specific situation, you with spring attack against an immobile foe without reach or special abilities, you can simply say "I kill it" and it can't do anything in response.
At a move speed of 20 feet, you can reliably prevent creatures without reach from full-attacking you with melee attacks, provided you have an open enough environment through which to move. This is especially useful against foes that depend on their iterative attacks or if you have some once-a-round trick to avoid getting hit.
At a move speed of 30, you can reliably prevent many creatures with 10 feet of reach from full-attacking you with melee attacks, again provided you can move through the environment sufficiently easily.
I don't have an easily-searchable-by-move-speed table of monsters from 3.5, but my memory is that a move speed above 50 is fairly rare (dragons being the notable exceptions, followed by teleporters). At a move speed of 55, then, you can reliably prevent a great many - possibly even "most" - creatures without reach from attacking at all except via charge (and, with a little bit of helpful terrain or allies, possibly even that).
Beyond a speed of 55, you're going to continue to gain in utility from spring attack, but the increased benefit per speed bump will be more limited. You'll be able to stay out of the threatened area of more creatures, but the number of additional foes you'll be able to avoid decreases (eg., dwarfs have a move speed of 20; with a move speed of 45 on a featureless plain, you can prevent them from ever attacking you; increasing your speed to 50 doesn't help against that particular foe). Similarly, terrain will become less of a concern, though the theaters in which the extra speed will matter become less frequent.
The next jumps in utility happen at staggeringly high speeds, when you can spring attack from outside of ranged attack distances, though even a meager dagger's range is 50 feet, so your speed would need to be at least 110 to stay out of it outside of your turn (assuming the dagger's thrower couldn't move); a composite longbow's 100-foot range means you need to be able to cover 2010 feet per round to do the same.
There is no non-cheese speed at which spring attack's utility is maximized. Where is the minimum speed at which it's "worth it"? That depends heavily on the campaign. If you're mostly fighting in 30x30 rooms, a move speed of 20 will honestly provide quite a bit of the feat's benefit with extra speed going to waste. If you're fighting on infinite, featureless plains, more speed is always better.
But, what about multiple opponents?
Against multiple opponents, the analysis basically holds up the same; the additional opponents roughly equate to less-favorable terrain or combat theaters: you have fewer safe squares to enter, whether it's because a square is occupied by a foe or a stalagmite.