Must one inform the landlord? Can there be problems with sub-tenants?

You have to check your lease.
My lease clearly says I wouldn't be allowed to do it unless the landlord gives permission.

You can definitely run into problems with tenants like any other landlord. Because your name is on the lease, you'll be the one responsible for all of those problems, whether it's noise disturbances, unlawful usage, or damages. If your landlord lives on-site, you'd probably be wise to inform them, even if it's not mentioned in your lease. They're apt to notice the new tenants and probably be very irritated that they weren't told in advance (even if it would have been fine by them).

I would not recommend subletting your apartment. My wife did this before we were married, and the people practically trashed the place, and broke some of the appliances. They tried cleaning up, but my wife still needed to hire a cleaning person.

Yeah definitely not a good idea. What's best is to pay to get out of the lease at a lower rate since you can provide a replacement tenant. Some landlords are pretty cool about that kind of thing.

Sometimes one has no choice, couldn't one make sure to get good sub-tenants?

How does one ensure that???

If you were a private homeowner, you would want make equally sure that the person you rented your apartment to was a responsible person. It may be even more important because a private homeowner may actually be living in the same premesis as the renter.

Screen a prospective tenant by using a tenant credit service, have the tenant fill out a questionaire about place of employment, previous places of residence, family members who will live in the rented apt., etc.; and make phones calls to double check the info.