I have used unlocked routers and even unlocked a couple myself.
Some ISP's (BT for one) choose to lock some routers to their network so that they don't work on another ISP, hower, there are ways of unlocking most of these.
Some come as new but are unlikely to have warranties as they are usually supplied to another user so are really second hand.
The performance depends on the model really. One example is the BT Business Hub whichh comes as part of a BT Business Broadband deal. These routers are known to handle long lines well and are very reliable. Some companies choose to substitute these with their own for whatever reason so sell the routers on. They sometimes come locked so the buyer must do the unlocking themselves or end up on Ebay or similar already unlocked by a third party for a slightly higher price.