To get to the bottom of this, we performed one of our famous Question of the Day call-arounds, and were surprised -- in some instances shocked! -- by our findings.
We contacted the front desk at several Caesars Entertainment properties to get the lowdown on early check-in fees and found that each hotel we spoke to had a different policy, although standard check-in time was consistent at 4 p.m. Here are some property-specific charges, however:
It's not just Caesars Entertainment properties in Las Vegas, either. We called Caesars Atlantic City and were told that an early check-in fee applies on any weekend, while on weekdays it depends on available inventory. The fees range from $25 to $50, depending on prevailing occupancy rates (they were pretty vague and we couldn't pin them down to specifics). If you want late check-out, which is officially at noon, it will cost you another $50.
As to the second part of your question, MGM Mirage Resorts does seem to be somewhat more flexible and accommodating (although, as noted by this questioner, they get you with those mandatory resort fees, which vary from property to property).
A call placed to the Mirage, for example, (where the nightly resort fee is $28, but includes more amenities than many) informed us that if you book online and select the early check-in option, then an automatic $30 fee applies; however if you don't specify that option when booking, then subsequently decide you need it and call 24 hours in advance of your arrival, they will waive the fee. Huh? We guess the moral here is to not book an early check-in unless you absolutely have to, and then wing it and hope for a freebie. Check-out is officially at 11 a.m., but the hotel will normally give you an extension until noon, gratis; thereafter, the fee depends on occupancy levels, but it'll likely be in the region of $30. Guests are advised to call in advance if they know they need a late check-out, since the rate can vary from day to day, depending on how busy the hotel is.
Calls to Red Rock Resort and Hooters, both of which also charge a resort fee, allow for complimentary early check-in, inventory permitting (official arrival time is 3 p.m. and 4 p.m., respectively). The Cosmopolitan, however, is even more accommodating: Official check-in time is anytime between 3 and 6 p.m., but if you arrive prior to that and your room is available, you may check-in early with no fee. Check-out is officially at 11 a.m., but they'll normally extend that until noon for no charge, so long as they have available inventory. Thereafter, you get into the half-day-rate scenario, but that again depends upon how busy the hotel is. On top of this, the hotel does not charge a resort fee**. [See ADDENDUM in "Update"]
So, if you think you may require early check-in, we definitely advise checking with the hotel front desk before you make a booking, or you might get stuck with an unexpectedly hefty fee. As far as late check-out is concerned, policies seem to be more flexible, but fees will likely still be incurred. If you don't require the room, but simply need somewhere to store your bags prior to departure while you take one more shot at the tables, you should be able to leave them with your hotel bell desk free of charge (aside from a toke).