It's one of my favorites. The 251 is a great compliment to it, so I recommend the combo. It seems more than lacking low end as compared to a Neumann, it's a tad brighter, and has a different proximity effect. I know it's splitting hairs, but the old U47's I've used have an exaggerated proximity effect that reaches further than most other LDCs.
So what ends up happening is kind of application dependent. If you've got a singer that's kissing the microphone, you definitely won't have a lack of low end. I've yet to try swapping the tube with a NOS something or other, but I have the feeling that would be the icing on the cake, as the L47 is a tad harsher on the top the an old mic, but I suppose that's to be expected.
It's eff'n huge on FOK, or bass. (upright, or amped.) Sometimes too much so, so I swap for the fet body.