My son (2.5) has been going to a 1 day/week preschool program this year. It is not as expensive - probably $250 for the whole semester, but it is only 2 hours a week.
It has been interesting. There are some good things - he gets "socialized" with other kids and other adults, starts to learn the ins and outs of a "school type" setting (which they don't necessarily need, but is good practice if available), etc. The down sides are like you mentioned, that it is only one day a week and I find that my son is VERY outgoing at home with us or when at family houses (grandparents, aunt/uncle/cousins), but is quite shy at school with people he is not that familiar with. I think it is good for him to be there, but also wish it was more often for his "comfort level" sake.
During this time, I sometimes have younger kids in tow - I babysit. But even with them along, I still find the time relaxing as I can go to the store (cleared with their parents of course) or when it is just me, I relax at the store or go to an exercise class, etc. Even though the time is short, it can be rejuvenating.
I think a lot depends on the personality of your son too. How is he separating from you? My son has always been really good. Some weeks are harder than others, but I think it is good for him to be under the direction of other adults other than just me - he doesn't go to day care at all so this was my opportunity to get him exposed to other things.
I am an early childhood guru so I give him a lot of that stuff at home, but I can't give him everything - the extra kids, different adults, etc. So, I thought that was a benefit for him. Plus, it gave me a little break.