I'm only doing a Certificate in Business Studies with the OU, but I've done a BSc and a Diploma in Music with them in the past and can't rate them highly enough. I'd like to do a law degree but I simply can't afford it (and as a twice Bachelors holder I hardly qualify for a free place).
I can only say that a degree from the OU is as highly regarded, if not more so, than one from any other university. Their courses are up to date and use the latest technology and teaching methods - for instance some of their business course tutorials are held via video conferencing, a valuable business tool.
OU graduates are known for being committed and dedicated, and many companies and public sector employers use OU courses for their staff.
If you're already registered with the OU, go on to their First Class system and ask a few people in the forums there. If you're not, then you'll find many of the course pages offer feedback from previous students, good and bad.
Better still, why not start with a short course that can be counted towards a future degree? You only need to commit one course at a time.