What was the work and life like for an average employee of the Roman postal service, Cursus publicus? How dangerous was the job, how much oversight was there from above and what kind of messages were being delivered? Also, would a messenger be regarded with general respect?

First, a very quick background of the system in the Republic. In the Republican period, when the Romans were still conquering Italia, officials sent from Rome on government business had a right to extract supplies from the Italian sociii ('allies') for their journey. This could have been food, lodging, horses/ships, and we get the impression that they usually sought out the local aristocrats of whatever Italian state through which they were traveling (they didn't steal the one horse of some poor Picentine farmer). This was dediticium. Things got more complicated when the Roman frontiers pushed beyond Italy. These same Roman messengers on official business came to have the power to summarily demand supplies from anybody -- they were issued a diploma which was binding on all provincial inhabitants. Inevitably, abuses occurred. Provincial enclaves began to complain at Rome, and several regulator laws were passed (a lex Porcia is known about, but the specifics are lost). Within that system, there was a special privilege called the libera legatio which could be bestowed on anybody, not just those traveling on Roman business. This was almost always given to important men and could last up to five years. Cicero during his consulship (63 BCE) tried to limit this particular flavor of Roman-sanctioned travel requisition (de Leg. 3.8, 18) but it was overturned again by Caesar. In general, the holders of the diploma or of libera legatio were men of status, though it is hard to know for sure. Certainly none of them were slaves or the like. They were not professionals, either -- they were specially selected to bring this or that message, or negotiate this or that contract, or whatever. They were the businessmen of Rome, flying (riding) on Rome's expense account (except it was the people along the way that actually paid). You get the idea.

In the Late Republic, there was another class of messenger, just as loosely-organized, tasked with communicating with provincial governors, generals in the field, or foreign states. These were frequently trusted slaves, freedmen, or a group of semi-professionals called the stratores ('the saddle-men'). In addition to these, we hear about a group called tabellarii ('letter carriers'). Some (but not all) of the tabellarii were attached to the publicani, who were men of the equestrian rank involved in the machinery of collecting tax revenue from the provinces. The negotiatores (um, 'wholesalers') also frequently used these tabellarii.

In summary: an official system in which a person (usually of rank) could travel quickly, taking what he needed along the way via Roman authority; a more loose system of professionals and semi-professionals which served both governmental and private sector needs, without the mandate to requisition from points along the way; and to this we should add all the private couriers that kept private correspondence flowing. Cicero made use of anyone he could find -- his slaves, business contacts, fellow aristocrats -- whoever he could trust. If other elite Romans were sending as many letters as he was (and we think that they were), then there were letters crisscrossing the Mediterranean at all times by these incidental ad hoc couriers.

Now to your question. All of that mess was slowly systematized during the Principate. Augustus formalized the stratores (aka speculatores) along the major roads of the Empire, and established at least some of the waystations (mutationes) we hear about later along the routes. Interspersed with these were constructed (or augmented) mansiones ('travel lodges' or 'stop-overs') for staying the night, getting a hot meal, et cetera. Like in the Republic, the responsibility of paying for and stocking these stations fell on the locality in which they were placed. Only those with the diploma could make use of them, though. I don't know this for sure, but it was probably the case that anybody could use the stations, but had to pay, and if a diploma-bearer came along and there were not enough supplies because a local had bought it all, that was probably unpleasant for all involved. Italy was removed and then reinstated to this responsibility, and there is some evidence that Hadrian "nationalized" the whole thing to relieve the burden on the provinces (Spart. Hadr. 7). We still hear about the hardships on the cities of maintaining the cursus much later, though, so...

There were six to eight stables along the route for each day, each with animals necessary for whatever conveyance was being used - horses or oxen for carts or donkeys or whatever. There were also (probably local) support personnel like drivers, vets, grooms, and guards (vehicularii).

No early Empire author goes into specifics, but from inscriptions we get the impression that the couriers were all Imperial freedmen -- that is, slaves who had been freed by the Emperor and were generally loyal to a fault. As far as status, they were more important that normal freedman, for sure, but still freedman. The Imperial freedmen are a special class in the Principate and probably deserve their own top-level question! The person in charge of it changed name as time went along: under Trajan it was the tabellarius a vehiculis and his 'secretary' the tabellarius a commentariis vehiculorum. Under Hadrian, we hear about the praefectus vehiculorum, who was probably an equestrian rank and not a freedman. One inscription (CIL 6.1598) mentions the impressive praefectus vehiculorum a copiis Augusti per viam Flaminiam, which suggests he was responsible for transporting food for the Imperial army along the Flaminian Way. Still later, under Septimius Severus, we hear about curatores viarum which seem to have something to do with it all as well.

The emperor of course had the right to issue a diploma document to whomever he wanted, but other officials also technically had this ability (the right is called evectio). Consuls, provincial governors, and even the Praetorian Prefect could do it when wanted (presumably with Imperial consent). For the emperor's personal correspondence, there was a special corps of messengers called the tabellarii diplomarii, and they superseded all others at the way-stations.

The cursus in its full swing (let's say 2nd century and later) was quite a thing. There were a host of different kinds of travelers using the system. All of them together were called veredarii, because they rode veredi (light horses, not the bigger war horses). Among these travelers were the emperor's horse-couriers (stratores); intelligence officers (speculatores); the frumentarii (having to do with the wheat supply to Rome); and, later, the agentes in rebus (no idea what they did). In addition to all these were random others holding diplomata. A person using the cursus could take with him a socius (friend, or slave).

As to the danger: if the traveler was conveying goods (in a wagon or whatever), there were two or three official guards attached (custodes). This suggests that the roads, at least out beyond the cities, were not exactly safe.