This is a fairly new allocation for short-range communications, apart from one wall-mounting "base" unit for use at home or office - all the PMR446 units are hand held walkie-talkies.
They range from positively tiny to really heavy-duty professional kit, and cost from as little as £50 a pair upto nearly £200 EACH.
Being on UHF (446 MHz, as opposed to CB on 27 MHz) it is not compatible with CB sets - CB’s and PMR446’s cannot talk to each other. This service has some advantages over CB, but also some disadvantages...
There are only 8 channels, as opposed to CB’s 80... The maximum transmitted power is .5 watt, compared to CB’s 4 watts... You cannot connect an external antenna to the units, so the shielding effect when used inside metal vehicles cannot be overcome. This effects the CB handhelds more than it does the PMR units, but nonetheless - do not expect more than a mile or so from within your vehicle, especially with the cheaper units.
Despite the seemingly low transmitted power, for handheld-to-handheld communications, PMR446 will actually out-perform CB in terms of range (it’s all down to aerial efficiency!)... It is license-free, whereas you should have a CB license... There is a huge range of units available, some of which are extremely small - much smaller than CB’s will ever be.
Some companies will claim over 300 “channels” for the PMR446 units, but this is incorrect. You have 8 channels (different frequencies that you can transmit and receive on) but there are ways of making the receiver select who to listen to, so that you do not hear any other users who may be on the same frequency - this can give you over 300 combinations. You can set the units so that they will only talk to other units with the same settings - and there can be more than 300 of these "settings".
Having said this, there are only 8 channels, and if someone else is talking on the same channel as you and the person who wants to hear you is receiving the other user at about the same signal strength it’s likely that you will block each other out and nothing will be heard. This will only become a major problem in areas where there are a lot of users because by the nature of the 1-3km (1-2 miles) range, you will need to be fairly close to them to get problems.
This is only really going to be a problem when stationary or on foot, and it's only at large motoring events that you find find the PMR446 channels congested enough to bother groups of people in cars quite close to each other.
The potential problem is made worse by the fact that most of the "professional" users of PMR446 (garden centres, shops, factories etc) do have their settings in place so they have no interuption to their work communications, but many people using it for contact with their mates, or while out with the kids or shopping, hiking, etc DON'T bother to use these settings, so they are receiving everything that's on the channel...
The net result is that you can hear B&Q's or Tesco's trolley collecting team (for example) but you cant get through to them to say they are disrupting your conversation by "walking all over" the signal from your friend! As i say, not really a problem for those in vehicles who will soon be out of range.
PMR446 can actually be so flexible, ideal for recreational activities as well as for professional uses that many people have PMR’s for family communications, holidays, outings, keeping track of kids etc - irrespective of whether they use CB for other things or not.