In Australia, CB radios first appeared in the '70s as 4 watt 23 channel AM operating in the 27 Mhz range that were designed for the US market.   In 1977 they were legalised in Aust and were now 18 channel 4W AM and 12W PEP SSB.   Three years later they became 40 channel units.
The Aust government had introduced licensing fees and call signs but this was largely ignored by users and was soon abandoned, probably due the cost of enforcement.

Later, in the '90s, UHF FM CB radios became available that offered a number of advantages over the AM types.  These radios operate in the 477 MHz range and power output is limited to 5 watts.
 

27 MHz AM radios have a range typically around 10 - 15 Km and 25 - 35 Km in SSB mode.

Due to the signal reflecting off of the ionosphere in instances of high sun spot activity (it varies on an approximate 11 year cycle) ranges of thousands of Km are common although very unreliable.   This phenomena affects the entire HF band (3 - 30 MHz) to varying degrees.

Because of the nature of amplitude modulation, atmospheric and man-made electrical interference can be a major problem with the ability to severely limit the above distances.

Although there are times and circumstances where 27 MHz SSB will out perform 477 MHz CB radios (they tend not to be quite so line-of-sight dependent due to the lower frequency), on balance I would suggest that you give the 27 MHz radios a miss in favour of the UHF radios.

But I still retain my pair of Uniden PC122  27MHz SSB radios (shown on the left) from the 80's.  A pair of these using 5/8 wavelength antennas are good for 35 Km.
Given suitable atmospheric conditions, brief interstate and occasional overseas exchanges were a regular occurrence.   Sunspot Cycle 24 should peak in 2010 so these conditions are with us again.
 

These UHF radios have 40 channels (but only 38 are to be used) and a range typically around 15 - 25 Km for a 5W unit.

Range depends on topography. All UHF ranges mentioned on this page are for open country and near line-of-site, in other words, as good as you can expect.   In hilly country, range will progressively decrease.   In built up areas, range will typically be halved or worse and in cities, don't count on much more than 1/10 of that range.   Mobile to mobile is assumed, mobile to repeater can be double.

Because of the (high) frequency, signals are not reflected from the ionosphere and being frequency modulated, they are for the most part, unaffected by electrical interference.

Duplex Operation
This type of radio has a lot of very useful features not found on the 27 MHz radios, one being "duplex" operation (I prefer to call it semi-duplex).
This enables you to use repeater stations located around the country to greatly extend the communication range to 100 - 120 Km.

Contrary you what many think, you are not transmitting and receiving simultaneously. 
A repeater is nothing more than a transmitter and a receiver located in an elevated position where the receiver output is fed into the transmitter.

When selecting duplex mode of operation, you are using two of the forty channels, one to transmit to the repeater and another to listen to the repeater.
For example, the UHF emergency is Channel 5, if there is a Ch 5 repeater in your area, select Ch 5 and then switch to "Duplex". 
The Ch 5 repeater is designed to receive Ch 35 and re-transmit on Ch 5.

Now when you transmit you will be doing so automatically on Ch 35 (even though your radio is still showing Ch 5) and the repeater will receive your transmission and immediately re-transmit your signal on Ch 5 so anyone listening to Ch 5 duplex (or Ch 5 simplex) will hear you via the repeater.

Because of the elevated position of the repeater on a hill top, tall mast etc, 50 - 60 Km transmitting to and receiving from the repeater is not uncommon, giving a potential 100 - 120 Km  mobile to mobile range.
 

Most people would automatically purchase an in-vehicle UHF radio but a handheld is a very capable and convenient alternative worth considering particularly if you are not keen to fit an in-vehicle radio and antenna.

Most handhelds in the 1W  to 5W range have all the features of in-vehicle ones and  3 - 5W handhelds aren't too far behind in range either.
The reason being, with in-vehicle radios, there is the loss of the coax aerial feed but more importantly, deterioration of that coax over time (damaged, water logged) and often poor antenna match (high VSWR).
The handheld on the other hand has a factory optimised 1/4 wavelength antenna not subject to degradation and of course no feeder loss.

I have measured transmitter power at the end of the coax a low as 1W, similarly, I have taken arbitrary field strength readings that have shown ERP (effective radiated power) of a 5W handheld to be not too far down on many in-vehicle ones.   The result is not such a big difference in ERP as you may think.
Some of the high end 5W handhelds are waterproof too.

If your need for a UHF radio is to keep in touch with traveling companions on the road, a 1W handheld would give you a range of 4 - 5Km in open country with both handhelds operating inside a vehicle.
My own handheld, a 1W Uniden UH049SX, used outdoors in open country has a range of better than 10Km working to my in-vehicle Uniden UH-100 with a 4.5db antenna.

Personally, I am sold on the convenience and portability of the handheld much in the same way as I am with my laptop vs my desktop computer.

If you are in the market for a UHF radio, best to stick with brands like Icom, GME Electrophone, Uniden and Vertex and if it is a handheld, choose one that uses rechargeable AA batteries, if possible, rather than proprietary battery packs.
Some handhelds have quite a few optional extras including in-vehicle mounting arrangements.
 

A word of warning, although no where near as bad as in the 70s and 80s, there are still some pathetic morons on UHF, usually in the cities and on the repeater channels who amuse themselves by disrupting other users with their childish antics.
Totally ignore them, resist the temptation to even acknowledge their existence.    Never ever reply in any way, if they think you can't hear them (they're not very bright) they will find something else to do.