I mostly patronize independents but I'll give you the benefit of my half-dozen or so encounters with higher-profile agencies.  The ones I've been to are exactly as you describe.  Don't count on showering before or after.  You get exactly the time you pay for and not a minute more (or less).  Generally there are at least three girls available, so WYSIWYG.  Yes, they are safe because to be otherwise would be bad for business.  The first time with an agency you'll be screened.  This can be quite mild or quite stringent and intimidating depending on the establishment.   Nominally, you'll have to provide your first name and work phone number.  At the other end, you'll be asked for a paycheck stub, photo ID, you'll be frisked and your cell phone held until you leave.  Payment is left with the girl either before or after the session.  I generally volunteer it and say something like "can you take care of this envelope for me?"  Overall, agencies are like any other business -- they like repeat customers.  The second visit to one is generally better than the first, since by then you're a known quantity and will be treated as such.