If you like the sound of the UA 610 preamp, then that would be a good choice. If you have no clue about the UA 610 or any other high end type of preamp, then you owe it to yourself to get a hold some and try them out. No one but you can tell what preamp you like.

Buy it from a place with a great return policy, put it through it's paces for as long as you need(but before the return policy runs out) and if you like it keep it, if you don't return it. It's nearly impossible for us home recording guys "get ahold of some(preamps) and try them out" without actually purchasing the item first.

It's nearly impossible for us home recording guys "get ahold of some(preamps) and try them out" without actually purchasing the item first

No, very possible. Dealing with a place with a good retrun policy is one way. You can also rent what you want and/or book time in a studio that has what your looking for. A good pro studio would likely have several different high preamps to choose from.

I have an la-610 and it is great on vocals and accustic guitars,though it warms up the guitar and you may want a cleaner/more transparent sound.
would'nt be bad to have a transparent pre as well.I use a neve portico 5016 and a mackie 400f for cleaner sounds but the UA is great.

Actually just to further AudioGaff's point, I am a small studio guy and I have no problem borrowing from my local shop. Now they know me pretty well, but from there viewpoint its good businesses, as I will likely purchase something in the next 6 months, and when I do guess who will get my dollars. Get to know your local retailers you would be surprised how generous they can be. Keep in my though I never expect any favors, but they continue to impress me.

I use a Rode NTK w/ the LA-610 preamp/comp, it really shines. I'm a bit partial to UA stuff, just from my experience(although limited) and from what I heard. But I have had the opportunity to try a few preamps, and the 610 is one of my favorites. It's dirty, but not muddy or dark. It has delightful overtones, very strong presence. I use this preamp for pretty much everything, and although it would be nice to have some variety, but I simply can't afford anything else right now. Here is personal break down:

Vocals

The 610 by far is best on vocals, it's quite creamy and when you boost the gain you get a slight distortion, very similar to Sinatra, Beach Boys, etc.

Piano

Strong distinctive presence, I find there are dramatic differences with other microphones. Very bright, but dependent on where you mic it and how you boost the level/gain you can get a very dark/scary sound as well.

Acoustic guitar

This is nice, good sharp clean sound, if you have an EQ you can boost the hi's and get a very distinctive treble acoustic, very similar to the Kinks and Beatles. Sometimes can be a bit "roomy" which can be problematic if you want more of a direct sound, close miking works very well, I find this mic preamp really captures room sound, which sometimes is awesome. But having a different legion of microphones to choose from helps.

Electric guitar(dist)

This is where I find the 610 really doesn't shine. Even with a compressor, it can sound uneven and tinny. I've tried various guitars, amplifiers, and microphones including a Fender Telecaster, Rickenbacker 360, Danelectro 12-string, Silvertone Jupiter, and a Gibson Les-Paul through a Vox AC30 and a Fender Deluxe And although with various tweaking I was able to achieve some pretty good sounds. It wasn't ideal. IMO.

Electric guitar(clean)

Now this suited the preamp much better, very balanced and w/ a bit of reverb is extremely well rounded. Good, but not as good as acoustic.

Bass

DI is incredible, at first I was skeptical, but after allowing the bass to sit in the mix, it was so punchy and clear. Dependent on the bass rig you have, micing the bass produced different results, equally as good as the DI.

Drums

Not as clean as one would expect, if you are looking for modern dry drums sounds, I don't suggest this mic preamp at all. It's full of acoustics and warmth, but not distinctive. Drums never seem to stand out i the mix even when cranked, but if you are looking for this kind of drum sound(like I am) it's just right.

I hope this helps, I always thought it would be nice to have a breakdown on this stuff. Although I would like to stress, this is my personal opinion. I'm sure others may disagree.