I've only made reservations at lunch for large parties, mostly for the folks on this board! Otherwise I've just walked in.
MYTHOS is by far my favorite restaurant in Universal Orlando. Lombards Landing is my favorite in Universal Studios. In Portofino I've only done breakfast at Trattoria del Porto which was AWESOME. I'm still determined to do Mama Della's but still haven't made it there.
As for the perks of being an onsite guest, this is from our usoinfo site:
1) Universal ExpressSM Ride Access at both Universal Orlando theme parks
2) Free onsite transportation to and from the theme parks and CityWalk
3) Priority seating at select Universal Orlando restaurants
4) Package delivery to your hotel room
5) Use of hotel room key card to charge purchases
Have a great trip!