Currently most of the educational data-loggers on the market are single-source; one company makes the hardware and software, and if it runs into problems you're out of luck.
My school made the mistake of buying this stuff from a company which initially seemed to provide a very good product. The first batch of equipment we bought worked perfectly and seemed to have some advantages over its competitors. When we placed a VERY large order a couple of years later they proved to be selling stuff that was partially the hardware equivalent of vapourware (e.g. they were selling it before they had production running properly) and partially in very short supply. It took a YEAR to get everything.
About 18 months later the school migrated from Windows NT to Windows 2000 and suddenly nothing worked. After considerable argument this ended up with us buying another version of the software (price £250) plus site licenses (another £200-odd). That worked pretty well, apart from it occasionally forgetting its registration key.
Now we're rolling out Windows XP and (surprise) we ran into problems with the software. It now turns out that the manufacturer of the hardware and the company that produces the software are on the outs, and the software company will no longer support copies of their program that were sold by the original hardware company. Their solution; pay £250 for the latest version of the software. Plus loadsa money for more site licenses, of course... Or buy their dataloggers, which come with the software free - but of course we don't actually need more dataloggers, all of our probes etc. would need expensive adaptor cables to work with them, we'd still need the site license, and we don't actually have the money anyway.
So we kept trying things, and after a lot of messing about I discovered this morning that the problem only occurs if you log on as a guest, rather than as a teacher or pupil. With our 2K rollout this didn't matter so everyone used guest every time, with XP it apparently does. Problem solved, and I suspect that they could have given me the answer in about two minutes if they hadn't been trying to pressure me into spending lots of money.
The sad part of this is that the new range of hardware the software company makes is actually rather nice, and if we ever have some money again we might have looked at it. But everyone who has had anything to do with this is so put off by their attitude, both this time and on the previous occasions that we've dealt with them, that we're not going to touch it with a bargepole. The hardware bods, who are also a major player in laboratory equipment, have lost nearly all our business for similar reasons (plus gradually becoming more expensive than their competitors).
I'm not going to name names because it's possible that others have had more pleasent experiences with these companies. But my experience has been unending delays and excuses from the original hardware company, and a combination of rather inept hard salesmanship and unhelpfulness from the software company. The odd part is that both companies continue to thrive, proving that if you are selling to a niche market you can get away with crap that would kill companies selling to the general public.