Having used both unmanaged and managed switches I would now never want to use an unmanaged switch again. Where I work (a school) the policy used to be to do things as cheaply as possible, i.e. buying unmanaged switches, etc. What we've found is that buying the cheap stuff only ends up costing us more in the long run. You buy the cheap stuff and then a year or two later someone wants to change something in the network and the cheap stuff isnt capable of handling it so you have to buy something new. If you'd bought the more expensive stuff to start off with the second purchase (and the time required to install/configure it) wouldn't have been necessary and money would have been saved.
We now use managed HP Procurve switches all over (about 15 in all), they've been very reliable, HP's warranty is superb and no quibble (lifetime next day replacement). The thing that really makes the decision easy to justify is the fault finding on them. We installed the new managed switches and quickly discovered that a fair bit of our CAT 5 had been badly installed and had been causing major problems on out network. We had already manually debugged part of the network, which took hours and hours, the time taken to debug the rest of the network with the HP switches was minutes (we did the debug a couple days after installing the switches in order to make sure the switches had had time to see traffic over the cabling).
The cost difference between managed and (good) unmanaged switches is not huge and if the managed switches highlight 2 or 3 problems with your network they will have probably paid for themselves. Remember decent managed switches will offer features like VLANs, Port based Authentication, SNMP reporting, Multicast filtering, broadcast control, and lots more. You might not need the features straight away but you'll need at least some within the lifetime of the product. I assume you're buying switches with the intention of using them for the next 5-10 years.
One other point I would make is that I will never purchase 3com again. We have had nothing but trouble from 3com products. Their managed switches are far inferior to the HP ones (more difficult to use, do not provide the fault finding, not as good warranty, must pay for firmware updates, etc etc). I'm not saying HP are the best but for the price and small/medium networks they are highly recommended.