So the other day I was being my nerdy self and decided to contemplate how the battle of Reach may have gone had infinity and all of the tech/spartan 4s been completed and present during the battle. Halo nation (halo.wikia.com) claims that there were about 750 covenant ships involved, however, if I'm recalling correctly they did not all jump in at once. There were also about 150 UNSC ships and a super carrier present alongside Reach's smac platforms. I think that even with Infinity present Reach may have still fallen. Covenant losses would have been heavier for sure, but I'm not sure that even the badassery of Infinity could help against those odds. As the battle turned south I think they would've made the decision to pull Infinity back to Earth.
On the other hand, with a little (or alot) of luck the Infinity may have been just the thing to tip the scales. I imagine that it could be used to take on the larger covenant ships early on, and leave the smaller vessels to the rest of the fleet. The waves of covenant ships would be far less effective if they kept losing their carriers as they jumped to Reach. The spartan 4's would also have done a great deal to bolster ground defenses, possibly preventing the smac platforms from going dark.
Who knows. O: