the risk is high with bad capacitors , most dont get even checked and left to the new owner to sort out , i postet in past my experience with iMac G5's and ebay in the uk
most sellers even business sellers say "we never even heard about a bad capacitor problem in iMac G5's " but the very same sellers sell capacitor repair kits for the iMac G5
some give even warranties on their iMac G5 with bad caps inside , a german mate even found one on ebay and after buying it he found capacitors that had blown had been closed with superglue / solder , might last the 3 month warranty they give...
as changing the cpacitors is time consuming and you need really good soldering skills , so most avoid that process or only change the 1 or 2 that already blew up and leave the rest unchanged to blow later (happend to me)
other sellers just take the risk of giving you warranty for their iMac G5's they pulled out of some office in working condition and hope it will last another 3 month
but the "iMac G5 is slow" really depends what you want it to use for
it is certainly faster then any netbook on the market , even the G5 1.8 ghz model will play 720p videos , as thats the one i had
so if you want a iMac G5 from a collectors point go for the 2.1ghz isight model the last of the iMac G5 its the most reliable and most did come with good capacitors already (not all) , geekbench results are usually about half of a intel core duo with around 1200 , but its a single core processor , so it obviously cant deliver the performance of a dual core processor at the same speed , but slow is something different , for all usual tasks of the average user the iMac G5 is usable
i mean what you could do with it in 2005 you can still do with it in 2011 , but the realistic price should be closer to £200 or better below as that gives you the funds of a capacitor repair if needed and still stay under the price of a fully working iMac intel core duo
BTW those early intels make a great alternative to a iMac G5 , but only from a speed point of view
as those early intel core duo iMacs are even less reliable then the G5's with displays failing and gpu's and logicboards and psu giving up , as thats what you get sold on ebay for up to £400
and yes there are some really good ones out there who had apple care and got new parts during that period and had a caring owner who looked after the iMac G5 , and even had it checked and cleaned inside from time to time by a apple service techy
btw the same goes for the intel iMac's too sold on ebay , most are not even worth a quarter of the asking price , as they never got opened , never cleand inside, they are in most cases dust buckets waiting to die
so my optimistic guess is you can buy 1 out of 100 which is in serviced and good condition and will last the next 5 years without trouble , the other 99 are just good enough for spares , no matter if intel or G5 and no matter what age .as most are to scared to open a iMac and dont have funds to get them serviced , cleaning a Mac inside is not necessary apple said they dont need servicing ...ah no a misunderstanding apple said something about not user upgradable , but in reality the result is the same , and most sell them on to make profit before its to late to fund their new one and other sellers often sell them so you fund their home in the SW3 postcode of london
the high resale value is the reason for that problem, many disguised lemons among the apples,, and repairs are expensive on Mac's so once apple care ended buying a Mac is a big gamble , you got a greater chance in the euro million lottery