I don't like 18v tool sets - it is not enough for a circular saw, and barely enough for a recip saw, and too much for a drill.
The ryobi 18V set I have is junk - with the exception of the drill, which is very good and the sabre saw, and the light. The Circular saw won't make a cross cut on 1/2 inch plywood on a charge - the chainsaw was totally useless, and the recip saw is pretty marginal, although you might get a few decent cuts out of it. I've used dewault 18's and Makita 18's and although better than Ryobi - still pretty much gutless.
If you are a heavy user, go to the Milwaukee V28, or the Dewault or Bosch 36 volts. I tried each of them and I picked V28's and couldn't be happier - corded power and endurance in a cordless tool - but you will do OK with any of them.
If you just want decent cordless tools for DIY projects, I'd seriously look at the small 12 volt kits. Bosch came out with the first ones about 5 years ago, and Milwaukee followed with their M12's a couple years ago. I also noticed Makita and a couple others are coming out iwth them.
These tools are much more easily handled and stored than the big 18's or the even bigger 28's and 36's. 12v is plenty for a drill and impactor. Milwaukee also makes a couple of really cool other tools for the M12 - a small recip saw called a Hacksall and an inspection camera, and many others. You don't get a full sized recip saw or a circular saw with these small tools, but those are where I would go ahead and buy the corded anway. The only other trade off is in battery life - they do have to be recharged more often than my bigger batteries - but it's no secret those batteries are much smaller an lighter weight, and there's plenty of juice for most home projects.
One other piece of advice: take a look at the complete lineup of tools that you can buy for that particular battery type, and buy from the manufacturer that has a line that fits your needs, rather than a single tool . For example in the comparison between Bosch and Milwaukee compact 12 volt tools, I find there are pretty similar in ergonomics, power and usefulness, however the extended line of tools for the two are both different.