I'm a long-time Virgin Mobile customer and have been generally satisfied with their 300 minute service (grandfathered in at the unbeatable $25/mo).
Their iphone is a poor deal.
You're paying full retail price for the iphone, but from what I understand from Virgin's reps, they're locking the 4S's sim card. Unlike Sprint, which at least unlocks it for international use.
If this is truly the case, it's a severe case of "why bother". Buy a factory unlocked 4S for the same price and put it on straight talk's "unlimited" plan, which is cheaper than Virgin's "unlimited" plan and get faster internet too.
Plus, your virgin-only iphone will have almost no resale value. Unlike a factory unlocked iphone.
As much as I would love to replace my crappy Optimus V with an iphone, this doesn't make much financial sense on Virgin.