Shadow is a very, very strong PvP spec, and it gets even stronger with gear. Pros are immense damage that comes from your DoT and your cast spells. SW
is the strongest DoT that can be cast on players in the game
(excluding rolling ignites since that generally requires multiple mages), and with high enough shadow damage you can kill low stam classes with it alone if they're not able to heal/regen.
Weaknesses for Shadow are high mana costs on spells, very low range on Mind Flay and having to drop form to heal. You only have one spell that crits too, so "burst" doesn't come from crits but from being able to chain MB into MF and back into MB. It's easy to do if you have peels, but most of today's players know better. Make sure to have plenty of mana before entering a battle, and that you spec correctly. Stock up on water. Counters are SL locks that run with Fel Hunter (easy if they're not geared, hard if they outgear you) and I guess Hunters since they can spam Viper Sting and stay far outside Mind Flay range. LoS them if possible.
http://www.wowprovider.com/?talent=11215875_5_85d321z502322103511051
Basic Shadow PvP spec. There's 9 talent points I did not fill in because the remaining are optional talents. You can drop points into Imp. Mana Burn and Mental Agility, fill out Imp MB and Imp VE, go for Spell Warding, whatever you want.
Regarding Starshards, it's not really something I'd use unless I got locked out of Shadow spells, didn't need to heal and had plenty of mana to justify casting it. It's not that good. idk why you'd roll a non-Dwarf priest as Ally anyway. Fear Ward is just too good.