I realise that this is an old thread...but...why not. In case someone stumbles upon it via search engine like I did...
I am taking my instrument checkride later this month, so I'm not completely qualified to comment here...but I think instrument training was/is positively brutal. Much much more difficult than my private was, and has taken almost twice as long. As with anything, YMMV.
Now, to be fair, I got my PPL twenty years ago, and quit flying almost seventeen years ago. I decided to get back in the cockpit, get my BFR - sorry "flight review" - done, get a plastic license, and start instrument training. I'd never seen a gps in an airplane before, much less used one. Last I heard, Loran-C was going to save the world. So all of that weighs in on the difficulty, I'm sure. Probably the most difficult part was trying to figure out how to set up and use a Garmin 430 and 650. The UI on these units is horrid.
If you're reading this wondering if you should get your IFR ticket, the answer is "yes", in my opinion. As has been said above, it will make you a far better pilot in almost every imaginable way. But while some folks here thought it was "fun" or "easy" - and bless 'em all that it was - for me it was/is really tough.
Here's hoping the checkride goes well... <cross fingers>