I have a Leigh jig, which admittadly I am yet to use, but intend to in the next couple of months. I had a look around for a dovetail jig, and having had a bonus from work, i picked one up on ebay for £175 (i think). I went for it because the youtube demos made it look pretty straightforward, and I liked the way that it works. There are a number of discussions on here, and I remember one post where somebody listed a number of other things that you could do with the setup, that are not always apparent- trimming end grain for example. I liked the variable spacing that the jig offered, and less people seemed to be selling the Leigh because they couldnt get on with it than the Woodrat. I have no experience of the latter, so am not going to comment whether it is any better/worse.
There will be a learning curve with it, and setup for the first few components may take a while. After that, people that are familiar with them seem to be able to set up and cut joints quickly- again based on youtube and other demos that I have seen. The instruction book is comprehensive, and the vhs that came with mine would be most useful if I could have played it. Luckily, somebody sent me a copy of the files so that I could watch them on my computer. If you would like a copy, pm me your address and I will put them on a cd (or may be able to email them one at a time). It may help you to make a decision- youtube has a lot of demos, but these files show you how the joints are cut, rather than that it is possible!
Watch out on fleabay. You can get some good buys, but if there is only one jig on there, people pay a lot for them. There is not a huge difference between the D4 and D4R- if you so choose you can get the bits to upgrade. My D4 came with most of the upgrade bits from new (I bought it still in its box), so just because it is marked D4, it may or may not actually need a lot. The Leigh site will tell you what you need to get (if you want to) and what these new parts will enable you to do. Always worth getting in quick and asking the seller what they will take- if it is starting at £1 and there are no bids after a couple of days, they may well bite your hand off for £150 odd. Some sellers will do it, some will not.