I would agree to some extent; not totally.
I don't think the FP addressing is a huge issue, because IMHO most of the time it doesn't really matter if you get or don't get every possible FIR in there.
It is trivial to address it to the departure, destination and (some say this is unnecessary but I disagree, for some countries) the alternate. That's 2 or 3 addresses and they are obvious e.g. LFAT or whatever followed by ZTZX.
It is similarly trivial to select the FIR addresses from the pulldown menu.
It is the weird special cases (some of which are in the pulldown menu and some aren't) which have been apparently taxing people e.g. flight plans to Blackbushe having to be addressed to Farnborough also.
But I see no evidence that anybody actually cares about these. If the FP goes to the departure ARO then you can depart. If it goes to the destination ARO then you can land. If it goes to the FIR(s) in between then they will know about you as you fly enroute. Does Farnborough actually care about getting FPs for Blackbushe??? Both places are in Class G. This stuff has to be meaningless.
Especially as we are talking about VFR; 99% of UK pilots are not going to fly to Mongolia (or even places like Albania who may well get funny about some stupid detail); they fly to places like France who only just barely care and have never been known to turn somebody away at the FIR boundary because they cannot find their FP..
The
major issue with Afpex is the ~ 4MB application download. They have to stop this, and make it optional, using an announcement (on their website or with a pop-up) that a new version with new features is available. It has been said recently that this issue will be fixed soon. One can avoid the repetitive download by using the laptop's Hibernation feature, together with not exiting the Afpex application and leaving it running; this has worked for me for a couple of months at a time (until I forgot and did a full reboot and then had to do a total reload

). This download makes the system totally unusable over GPRS which is the major "mobile internet" system in Europe; 3G coverage is still poor in many areas.
The other issue with Afpex is the lack of feedback like SMS/email notifications of IFR slots, but this doesn't affect most GA pilots (cannot get slots in VFR) and IFR pilots have tended to be "mobile/internet" for some time because it is the only practical way to fly.
There are plenty of other "would be a lot nicer" things like why is it a java app and not a straight HTTPS website (which could be used in an internet cafe, etc), and there are loads of features in there which are not required by most pilots and which could be stripped out. There are some weird dependencies on the java runtime version although I have not been able to replicate these (I use Afpex only for FP filing and for AFTN messages for PPR/PNR purposes) but they would be due to poor programming practices.
I think that Afpex has dragged out a lot of pilots who have somehow managed - until now - to scrape through without using the internet, and in many cases without any preflight briefing whatever. The powers to be, having been running the Notam system wholly on the internet for 6-7 years, probably assumed that the GA community is by now up to speed with the internet, and moved FP filing to it too (making big payroll cost savings in the FBUs who in recent years were filing mostly just GA-VFR flight plans). I suspect they - as most of "us" - did not realise just how many pilots never use the internet, or (for the farm strip flyers) do not have the knowledge to purchase a laptop with mobile internet on it.
Afpex is actually a damn good tool. After you got the thing started, knocking up a flight plan is as fast as you can type it in (minutes), and it is delivered to all addresses within seconds of pressing the Send button. No messing about with filing something through some tower and then twiddling one's fingers for an hour while somebody hundreds of miles away manually re-types it into an AFTN terminal. For IFR flight plans it is brilliant and you get the ACK (or rejection) message back in seconds - nothing comes close. The AFTN messages can be used for PNR/PPR communications (they work slightly better than faxing); recently I planned a number of trips to N French airports and most of them were "Customs PNR". AND IT IS FREE

UK GA should rejoice
I have a homebriefing.com account too but use it rather less nowadays. Afpex is free, it is quicker to use, you get instant feedback, and the internet data usage is a lot lower than Homebriefing's which is pretty siginficant when using roaming 3G. Rather perversely, I use Homebriefing when doing short trips on which I carry a tiny EEE laptop/netbook which doesn't have enough HD space to support Hibernation.
Pilots are simply not trainned to address flight plans, and have little knowledge of it.
Very true, but they are not trained to do a lot of stuff which they need to know to fly usefully from A to B, like using the NATS website to get notams via the narrow route briefing (which is actually pretty good). GA flight training is pretty crap at the "operational stuff", at both VFR and IFR (IR) levels.