I plan on lvling my priest with shadow spec, then re-specing at 70 to holy and healing in instances and stuff like that? Do YOU think it's smart?

IMO, if you intend to heal, you're better off leveling with a spec that's closer to healing.  Sure, I leveled 1-60 originally as Shadow, and it's viable for healing at least any pre-60 instance, but I found leveling from 60-70 more productive as a Disc/Holy spec than I did 1-60 as Shadow.  By doing that, I was able to heal harder instances than I could have as shadow, not only getting me better gear sooner, but valuable experience.  By doing that, I became familiar with the talents, spells and, more importantly, the limitations of a priest as a healer.

Now, a number of people say that Shadow is the best leveling spec.  I'd agree, with the assumption that you're primarily grinding and not doing instances, and you're only concerned with leveling and not learning to play (like, say, you're re-leveling for different racials or for a different faction, or your guild needs you to re-roll NOW and learn as you go).  A Disc or LOLSmite spec sticks closer to a 70 Holy spec, while not losing a whole lot of damage, providing more survivability and longevity, and still making you a viable healer up to and including heroics and Karazhan (not really viable in SSC and TK).

Either way, I think it really depends.  If you enjoy Shadow enough to level that way, why respec as holy at 70?  If I wasn't really good at and I didn't enjoy healing, I wouldn't have remained Holy.

I totally agree with IZBadAzz.
I am a Holy Priest myself, and when I stepped into my first Heroic Ramparts, I had quite a shock.
You have to really know your stuff there. No time for indecisions, wrong heals or timing.
Fortunately, I had levelled to 70 as Holy, and my Normal instances experience as a main healer came to my aid.
There is a lot of subtle thinking for a pure healer (as with any other class, ofc), so you'd better become familiar with the usual techniques as soon as possible.

lol dude I am only level 20 priest on ravencrest but I am good but most importantly I know how to play the game.  I have beaten a level 29 in a duel and I am ballin with over 11g and I can get 5g like that *snaps ingers*.  Not bad neh?  Anyway,  O agree with you in a way becuase like what they said; If your only focused on leveling, go ahead but I think its best to choose holy or disc because you can get amazing gear.  I'm not a twink and  almost everything I have on me is rare.  BTW if you want amazing enchants at a low price and your on ravencrest add brotherjason to your friends list and PST me when I come on. ;)

First of all, there's no way you beat a 29 unless he didn't even try.  Second, your WOW Armory betrays your statement about "almost everything" being rare.  You have two rare items from Deadmines, and you even have two Common quality items, with the rest being Uncommon; that's hardly "almost everything".  Third, the enchants on your items are TERRIBLE... why would a Priest want +15 Health on Chest, or Armor on Gloves?  Fourth, and most damning, your spec is TERRIBLE, even for a level 22 Healer.  You SERIOUSLY chose Spell Warding and Holy Nova over Divine Fury, perhaps one of the single most important talents out there for a Holy or Disc Priest?