Helpful Information

Potentially useful information if you are considering a trip to Cozumel, regardless of where you stay --

Flights--

You can fly directly onto the island from the US, no need to go through Cancun.
Certain airports have better flight arrangements for getting you to Cozumel. For the western US, Denver, Dallas, and Houston are the best. Phoenix, LA and SLC would be next. The bigger the airport, the better your options.
Watch the travel times. Flights often leave on the date you select, but may take you 20+ hours to get there. From Billings (a very small airport), good connections can get you to Cozumel in under 8 hours, bad ones can drag it out significantly.
If you need to travel on certain days, it can sometimes save you both time and money to drive to Denver or another major airport, and fly from there (8 hour drive to Denver for a 4-6 hour flight is better and cheaper than a 24 hour airport marathon from Billings).
Google flights is a really good research tool for finding flights, but it doesn't list all airlines, and I've actually been able to find flights as cheap or cheaper using my credit card rewards system.

Travel on the Island--

Cozumel is a small island, but you'll need wheels to make the most of it. They drive on the same side of the road as the US, roads are in decent shape (although a little narrow in town), and traffic isn't too bad.
There is basically one town of a hundred thousand people, and then one road that loops around the bottom half of the island. As a result, it is almost impossible to get lost. Also, speed limits are low, and police are nice to tourists because they pay their salary. Take your time, use GPS, and you'll be fine. We've had a car on 4 different trips and never had any issues.
Car rentals range from $35 to $100 a day, maybe a little less depending on what you select, how busy things are, and how hard you negotiate. There are national chains, as well as a number of local providers, many of whom offer a pretty good deal, although with cars that are not exactly brand new. Wherever you rent, but sure and take a few pictures with your cell phone before driving away so you're not charged for damages you did not cause. You'll probably want to get the rental car insurance, Mexico can be picky about those kinds of things.
Taxis are available, and if you are planning to just relax around the resort, that might be your best option. They have set fares and are plentiful. Or rent a car for only part of your stay, and then relax for the remainder of your trip. However it works best for you.

Money Saving Suggestions--

After arriving at the airport, if you don't have a car reserved, avoid taking the official airport shuttles. They charge $50/person to take you to your destination. Walk 50 yards to the road adjacent to the airport and find a taxi. A taxi ride from the airport to our condo is just 20 minutes and costs $15 plus tip ($30 for a van) for however many people you can fit in.
If you don't want to rent a vehicle, a taxi will charge you $60 for 5 hours, regardless of where you go. They will wait while you're shopping, take you to a beach club, tour you around the island, whatever. And they usually know enough about the area to give you a good tour, recommend the better restaurants and give you a good feel for the island.
Haggling -- the smaller tourist stores are open to negotiating prices, and they have fun with it. Be reasonable and enjoy the process, but remember they have to put food on the table as well.
If you exchange currency before heading down that might save some headaches, but it might not save you money. Most places will accept US currency, but the exchange rate offered is less than ideal. That can also depend on the bank and the exchange rate they give you. Our most recent trip, the bank charged a fee and gave almost 10% less on the rate than the market. There are lots of exchange houses down there, and sometimes your credit card, if it is focused on travel, will give you a great rate and not charge fees.
Restaurants don't offer unlimited drink refills. They only serve canned or bottled drinks, so while you can get as many as you'd like, you'll be charged for each one.
Because it is an island, be aware that prices for merchandise run a little bit more than on the mainland, due to the extra effort it takes to get things there. That said, you can still find some pretty good deals.
If you get tired of The Money Bar restaurant, and you don't want to drive, have in town restaurant food delivered to the resort. Go to Fastydelivery, put in "Landmark Resort" for the location (unit 108 in notes), and then browse the hundreds of listed restaurants and their menus. They don't have every restaurant available, but they've got a bunch. The delivery fee is $4 or $80 pesos (plus tips--lots of tipping in Cozumel), and it will take about 30 minutes for your delivery, but it's a great option if you're not up for going out.

Thoughts on All-inclusives (FYI-our condo does not have an all-inclusive option, but most Mexican resorts do):
Pro’s--

Don’t have to carry your wallet around
The restaurants are within walking distance, so no travel required
There aren’t a lot of credit card transactions
They can be a good deal if you mostly stay at your resort, drink lots of alcohol, eat lots of food, and don’t mind a limited menu selection
It’s a safe choice in an unfamiliar area, you don’t have to make decisions or find restaurants

Con’s

Limited restaurant selection –usually only 2-4 actual restaurants at a resort (Italian, Mexican, Asian, Grill), plus a snack bar or two
Food quality is usually good, but not great (At home, how often do you go and eat at a hotel’s restaurant?)
Atmosphere is usually good, but not great
Restaurants are a part of a resorts operations operated by employees, not the point of the operation like a stand-alone restaurant
You are paying a lot for alcohol, whether you drink or not
You miss out on amazing local establishments owned and operated by talented folks who love food
No refunds if you eat elsewhere while away on tours, sightseeing, or other activities
Sur-charges on premium meals are almost as much as the cost to eat the same meal at a local restaurant
The incentive when you stay at an all-inclusive is to stay put and eat/drink as much as possible to maximize the value, then come home overweight and unhealthy.