Piranha has a very aggressive data pricing policy lately.
the only disadvantage is the billing increment (minimum charge), that's per 100kb, instead of 10kb of most international simcards or even per 1kb within EU.

I would still much rather pay 10p-20p per Mb on 100kb increments than 40p-50p per Mb with 1-10kb increments, mathematically not much of a disadvantage.:)

for small data sessions (eg email checking) you can end up paying more

I have to disagree, the maths do not support the argument. An average email including headers etc uses approx 50/75kb. So based on this, 1 email = 50/75kb @ 10p per MB with 100kb increments = 1p or 40p per Mb based on 10 kb increments = approx 2/3p.

most simcards (travelsim, camel, xxsim, airbalticcard etc) for EU roaming charge 0.39eurocents/mb billing per 1kb ... not 40british-pence per mb as you mention

they also have data bundles availiable.

0.39 Euro cents or £0.40 still mathematically more expensive and bundles are use it or lose it. its all about how much you use and as i have already pointed out the standard email is 50kb-75kb. I am happy leaving my data on knowing that worst case it will costs me £0.10 or £0.20 per Mb when travailing in the EU. and much cheaper outside of EU than the companies you have mentioned, but everyone to his own. My original point was they seem to be getting some very good deals in and outside of EU.

for EU data roaming (including Turkey, Switzerland and Albania) the best current solution is europasim (vodafone italy sim), 500mb/day for 3euro.
even if if you exceed the 500mb you aren't charged, you simply get lower browsing speed.

except the 500mb, you also get 25min. of outgoing calls & 25min. for incoming calls, plus SMS.

Agreed Europasim Card is best solution for Europe. Unfortunately most of my travel is outside of the EU.

For Canada I don't think you can do much better than Piranha. In fact somebody in my family uses it now as their main cell phone. Reason is the top up requirement (none) compared to the $100 top up per year of most Canadian cell providers.

Calls are very reasonably priced, $0.22 per minute Canada wide (again, most Canadian prepaid providers easily ask twice that).
Text $0.15 (Canadian providers: $0.50 per text)
Data is $0.37 per MB. If you don't want a data add-on with a fixed amount, the only prepaid provider Telus offers ppu data of $2.00 per MB. Granted, that is not in 100kb increments like Piranha.

I can't fault Piranha in any way though and of all the prepaid international SIMs I tried, it's the only one I kept. For the 2 countries I frequent regularly (USA and NL), I bought a local SIM (T-Mobile and Simyo, respectively) and in Canada I use Koodo prepaid.

But for extremely light users such as said relative, Piranha is actually a good local alternative here!

I hope she has a backup. Piranha may be all good for now. However, from experience, a lot of these companies come and go. As I posted in another thread, a year ago when I was Lagos, Nigeria, Piranha was useless. If it finally connects to the network, it shows emergency calls only. Thank God for my Ekit sim. I still have Piranha, and will give it another shot when I am back in Nigeria next month. At least to try to use up my credit.

If Piranha works for you, good then. However, always have a backup plan.