"your mileage may vary" depending on the rest of your system. What amp/pre-amp, speakers are you using? If your system is not a high resolution system and if you do not sit down in the sweet spot and critically listen, it may be difficult to appreciate the difference.
That said, I do have significant $ in my digital front end (Resolution Audio Opus21 CD player-$3500 list) and it was worth it for me. My system is highly resolving and I do sit in the sweet spot and do a lot of critical listening.
If I were in the market to perform a DAC experiment today (BTW, I have owned many different external DACs) I would buy a dACK! from Chris Own or the Scott Nixon TubeDac. Both are very reasonable at less than $400 new and both get nearly universal praise from those who have actually heard them. For more information about these two DACs, check out the forums ar www.audiocircle.com and www.audioasylum.com.
I might even buy a dACK! myself, just as an experiment. It is battery powered and comes with a charger. All indications are that is is world class, but at a real world price.