Am considering booking tickets using two different airlines--changing planes in Mexico City. Has anyone done this? Any trouble getting your luggage from one airline to the next? How much time to allow to clear customs, then check-in and re-clear security? Thanks.

Where are you going and what airlines are you flying?

Actually, it is done all the time. Just be sure that the two airlines have an "interline baggage" agreement so that your bags will be transferred between them (any agent can tell you that). Also try as much as possible to have both airlines' flights on the same ticket so that they know about each other and your journey is considered as a whole (thus ensuring various protections if flights are late or cancelled).

I've never been to MEX so do not know what the transit procedures are there.

If you do not understand NoFlyZone's answer, be sure to ask for more information. It is critical.

These forums are full of posts from people who booked two airlines separately on their own and then found themselves with no one to take responsibility when flights were delayed or canceled.

As long as everything is on one ticket or booked through a single agent or site like Expedia, you are likely to be fine.

You need to talk to both airlines.

The problem is that practically all airlines, except charter, non-commercial and most very low cost ones, in theory have interline agreements with each other. But this doesn't necessarily mean your luggage will be tranferred in every case.

Outside the US, the norm is that international transfers happen entirely airside: that is you don't go through immigration or customs. But, again in theory, the norm is that if your baggage isn't transferred automatically, you have to go through immigration, collect your bags and check in again.

But not even thois is certain. For example, British Airways generally won't through-check bags onto a non-partner airline. But on a recent flight where BA would have left my bags at Singapore, I simply asked the Singapore airport transfer desk to reroute the arriving bags onto my connecting SIA flight to Saigon. So, because of the second airline's helpful policy, I was able to transfer without going through immigration.

Beware, though, that airline staff are often very badly briefed about all this. On a more recent US-UK flight via Canada, US checkin staff and Canada-bound flight crew described the baggage transfer system at Toronto with perfect, 100%, inaccuracy. They even made a sponrtaneous announcemernt on the plane telling us to do precisely the wrong thing.

The only infallible guide these days is: pressure every airline employee you encounter to ensure your bag's through-checked. But organise your bookings and packing on the assumption it won't be.

Is you final destination within Mexico?

I used several airlines on a recent trip to Africa and Europe.
One issue I found was that unless all the flights on booked on a single ticket you may have to pay additional fees for overweight luggage. I flew to Africa on United/SAA and then took smaller African airlines in Africa and British Airlines within Europe.
The African airlines had a luggage weight limit of 20kg per person and British Airlines of 23kg per person. I was over and ended up paying additional fees. These can be quite substantial.