First off, there is limited time to get IPv4 PI space. IPv4 is running out fast.
Secondly, you should only apply for PI space if you meet the allocation criteria. Fortunately that is not too difficult to meet.
PI space is your own IP address block. It has been around for a long time, and anyone can apply. You normally apply via an ISP (as doing so directly is silly expensive). It costs the ISP 50 Euros a year. AAISP charge customers £5/month.
Once you have PI space, you can change ISPs as you wish, and keep the same IP addresses. This does limit the choice of ISPs to ones with "clue" but that is not such a bad thing.
The problem used to be that if you did not have enough devices using IP to justify a /24 (256 addresses) you got a smaller block. The smaller block was then totally unusable on the Internet. This was just plain silly and wasted IP addresses allocated like this. Thankfully RIPE changed the policy last year so that if you are multi-homing your IPs (connecting to more than one other autonomous system, i.e. ISP) then you can get a /24 even if you have too few devices to justify that size block. This means that PI space is finally useful for smaller networks.
Obviously people should not lie on their RIPE application, but it was, previously, very tempting for people to "over estimate" their requirements in order to justify a /24. The new policy should mean more realistic applications.
To make use of this new policy, you do need to have some routing to more than one ISP. This can be done, even on DSL lines, if you have the right ISPs. It is more common if you have something more like a "leased line", or perhaps our new FTTC Etherways.
Of course there is one slight snag. PI space is meant to be non-transferable. This does seem a rather arbitrary restriction to me. It is not a problem now, but when one can no longer get PI space there may be people who do want to transfer PI space. They will probably simply not tell RIPE this has happened, which means RIPE records are wrong. It seems far better if RIPE did allow such transfers.
However, there is a simple work around which costs around £25, and then £15 a year. Simply create a dormant UK Ltd company and request the PI space for the company. You simply have to have the company run your network. Then, if you ever want to transfer the PI space you simply transfer, or sell, the company instead.
Obviously the new user would have to still meet the requirements for the PI allocation, as you always have to when you have PI. This includes using the PI for your own use and not for customers.
Fortunately, while RIPE still do PI space, getting my PI space changed from my personal use to that of the new company "Thrall Horde" was relatively painless. I should have registered it under the company in the first place though.
Usually, with your PI space, you will want an AS number. This is an extra cost at present, but it looks like RIPE policy is changing and AS numbers will be free again in future. You can multi-home without using an AS - it is not a requirement to actually use BGP for IP routing, and even if you do, you can use a private AS which is then dropped when your peers pass on your route announcements. So for now you can save the cost of an AS number.
You can manage all of the RIPE objects yourself if you want, rather than your ISP. Well, apart from the route objects quoting their AS number if you do not have your own AS. This is pretty simple on the RIPE web site now. You, or your ISP, will need route objects and then probably have to arrange changes to filtering for transit provides, but this is usually pretty simple.
You, or your ISP, will need to set up reverse DNS, but again, this is not too hard.
So far the only actual problem we have found is getting geo-location fixed. I get loads of german adverts on web sites at the moment and cannot access iPlayer. We are working out how to solve that one now.
What is interesting is the huge difference between a new IP allocation via RIPE and a new telephone allocation via OFCOM. With RIPE it is possible to apply for PI space in the morning, get it a few hours later, set up all the RIPE objects, contact transit providers, and have full multi-homed working IP addressing on the Internet as a whole by the end of the day. With phone numbers it takes literally months to get the prefix manually added to tables by each separate UK telco and international gateway, and even then you can be chasing up exceptions for months more. Why can't the telecoms world be more like the networking world?