I agree with zappahead and Pugwash. I too started out with trialling V4.nn, then purchasing and upgrading (at cost) to V5.3 - I was tempted by the freebie inclusive DXO Filmpack offer. Not that I use the Filmpack - but you never know!
What has always impressed me with DXO, in all versions, is its optical correction abilities - they're fantastic. It always amazes me that a lot of folk put up with optically uncorrected images - the before and after correction images are a delight. The de-noising, vignetting and general correction is also great and DXO can turn my dull images into something worth looking at, for minimal effort.
I suffered a few install problems when V5 was first made available, but I now run V5.3 on Vista and it runs fine (it ran fine on XP as well, but I've just removed XP from my dual-boot PC to concentrate on Vista). However, when I've batched 360 imagesplus to process, it 'fell over' after 250. So now, I always run smaller batches of around 200, with no ill effect. I like the new interface, but I still have a lot to learn and experimentation to do with creating and enhancing my own presets. I originally was a jpg man, but now shoot raw because DXO makes it so easy to process. I like the batch conversion aspect because, as I work in IT, I sit in front of a couple of large screens all day and the last thing I want to do (as I have done for many, many hours) is spend all my evening tweaking every image and staring at another 2 screens at home. Nowadays, I stare at the 3rd screen instead - the TV - sad, but true!. Yes, I know, some images do need individual tweeking, but DXO has lessened this 'hands on' involvement for myself.
I would certainly recommend V5.3 and with a 15 day trial download being available (and the ability to extend the trial), you've got nothing to lose! I would definately recommend that your PC has the Microsoft .Net Framework 3.5 SP1 package installed prior to downloading the DXO trial, otherwise your install may 'hang'. The Framework download and install, on its own, will take about 30 minutes to complete.
Download and, hopefully, enjoy!