I've been to Isla Margarita a few times - in fact I returned from Margarita a couple of days ago.
The geography is absolutely stunning, and the beaches are generally picture perfect.
The dollar goes really far - I stayed in a lovely posada with AC, kitchen, pool & jacuzzi ......... for $20 a day.
On the down side ......... trash along roadsides and some street crime.
If you want a wacky fabulous trip to this area I suggest that you go to Margarita, rent a car and drive out to the Macanao section of Margarita island(western part of the island) - it looks like Arizona with mountains and red earth (really beautiful) - but the mountains plunge down to warm Caribbean seas, framing crecent beaches (most of which will be totally empty). There is a spa out there that is very clean, simple and a bit on the wacky artistic side - it's miles from anywhere and has what just might be the and massage treatment pavillion on the hillside overlooking the sea ......very rustic accommodations, but generally adorable and probably having 5 rooms max. Cost per person per day for room (double occ), three meals, massage and mud bath - 120,000 Bolivares (which on the black market is $45 USD. I can't find the brochure on it just now to give you the e-mail address, but I believe they have a website so you can probably search it out.
My favorite fishing-village beach is Manzanillo, with Zaragosa (Pedro Gonzalez beach) and Playa de Caribe following. Playa del Agua is popular and to my mind the best place to stay in that area is Costa Linda (lovely upscale single story compound with excellent food, pool, snuggly nooks and nice rooms. <www.hotelcostalinda.com
The island of Los Roques is reputed to be sandy streets, few people and great snorkeling (sounds like San Pedro)....... people rave about it.
If you go to Margarita, rent a car so you can explore. You don't need to reserve one - there is a rental-car place with about 12 agencies competing for your business, located just outside the Porlamar (margarita) airport.
Give them your credit card for a guarantee, but pay in cash (Bolivares) as you will have negotiated a huge benefit in the exchange rate that away. Last week the official rate was 1,900 Bs to 1 US; but you could get 2,500 Bs for a dollar from just about anybody wandering around the Caracas airport.
Fly to Caracas, then walk about 1/4 (don't take the bus or taxi) to the National terminal where you pick up Aeropostal to Margarita.
When you go to Los Roques you will take a boat from Margarita.
Oh by the way - gasoline costs almost nothing - I filled up the tank of a mid-sized rental car for one dollar US.
Other suggestions of interest - I have friends who were recently raving about an out of the way place called Little Corn Island off the coast of Nicaragua. population 700, two places to stay, lots of diving and snorkeling.
All that being said - I like Belize better. But when you live here, you need a break from perfection and a change of scene too. So..... when I go away I like Margarita for that - it's beautiful, gritty, exciting and a good chance to learn spanish.