That's an amazing price, I would jump on that right away. I bought two of those dual sets from American Musical last year for $489 a pop on an open-box special. That's a great price for a good set of mics.
The mics themselves are pretty nice, I've been very happy with them. I was a little concerned about their range since they have the integrated internal antennae and I was planning to rack mount them (you have to buy a separate shelf from Shure to do so) but they've done well. Audio wise, the PG58 is pretty close to the SM58 with some subtle differences. The biggest difference is in handling noise, the PG58's are definitely not as well isolated so as people jostle the mic from hand to hand or whatever, you will get some noise from it. Not a problem when singing but definitely noticeable if there's nothing else going on at the time. They do have the same proximity effect that the SM58 is known for which is great for adding warmth but you have to know how to manage it if you get a low singer who knows how to get right on the mic (just use the EQ to pull out a little around 250Hz to compensate if needed).
The only thing I actually don't like about them is that they use 9V batteries instead of AA's like all the other current Shure kits. An alkaline 9V will last a good 8 hours, I get 2 shows or better (depending on how much it is used) out of each new battery. I tried some rechargeable 9V's, they were good for a while but now don't even last through an entire show many times.
There is an adjustable level control on the back of the receiver for each mic. I had to adjust mine a little bit, the signal was awful hot with the factory config. Very easy to adjust, handy as hell to have that ability. There's no sync feature between the receiver and Mic for setting the channel, but it is an easy process to set them and it's hidden in the battery compartment of the transmitter so you don't have to worry about anyone accidentally or intentionally messing with it.
I've used the PG288 kits I have in a few live band situations where I was in a pinch. I was running FOH for a band that had a lead and 3 backup singers and given the nature of the stage, running cables to wired mics would have been a real pain in the butt. I gave the lead my SLX-Beta58 and the backups each used one of the PG58's. The results were good, I was really happy with how the mics performed.
Assuming you're using them for Karaoke, I would recommend somehow marking them so you know which is which. I bought the WA615M ID rings from Shure for this purpose and they work out pretty nice (although they do make it a little tougher to get the mics in and out of the clips if you don't know how to do it right):
http://store.shure.com/store/shure/en_U ... .105001600
Colored electrical tape would work too.