Barlows (both regular and telecentric) seem to be one of those tools that observers either are really comfortable having in their case or hate having in their case. I fall in the category of people that like having a barlow but I have no critiques for people that prefer not to use barlows.
The primary reasons I like to use barlows are:
1. They improve eyepiece edge performance.
2. They allow high magnifications to be achieved with longer FL, longer eye relief eyepieces.
The one thing I require of a barlow though is that its performance be essentially transparent with no negative impacts on image quality. In that regard I think the 3x TV is perhaps the most underrated barlow out there. I have had two of these 3x TV barlows and both were optically outstanding having zero negative impacts on eyepiece performance. I would rank this barlow a very close 2nd to the 2" 2.4x Vernonscope Dakin barlow. I suspect the 3x magnification factor scares a lot of people away from this barlow, but it is outstanding with 15-24mm 1.25" eyepieces and worth considering for anyone who likes using barlows. I use it extensively with my 24mm and 20mm ES68's.
Here is how I rank the barlows I have used over the years:
1. 2" 2.4x Vernonscope Dakin barlow
2. 1.25" 3x TV barlow
3. 1.25" 2.4x Vernonscope Dakin barlow
4. 2" 2.5x Siebert Telecentric barlow
5. 1.25" 1.5x Siebert barlow
5b. 2" 2.0x Siebert Telecentric barlow
Optically excellent but not flawless:
6. 1.25" 2.0x ES focal extender
6b. 1.25" 3.0x ES focal extender
7. 1.25" 2.5x TV Powermate
7b. 1.25" 5.0x TV Powermate
8. 2.0" 2.0x ES focal extender
Optically good but also overrated:
9. 1.25" 2.0x TV barlow
10. 2.0" AP Barcon
11. 2.0" TV Big Barlow
12. 2.0" 1.6x Antares barlow
Save up for something better:
13. 1.25" 1.8x TV barlow
14. 1.25" 3.0x Antares barlow
15. 2.0" 2.0x GSO barlow
16. 1.25" 2.0x Orion Shorty-plus barlow
17. 1.25" 2.0x Celestron Silvertop barlow
I know some people will disagree with my classifications of the various barlows, but that is how I rate them for my purposes and what my advice would be if you are looking to pick up a barlow. Obviously there are barlows I have not tried. However, the thing I see is that the 3x TV barlow is absolutely one of the best out there - only edged out by the 2" Vernonscope Dakin barlow. The only ranking on my list that I might like to re-investigate is the 1.25" 2.0x TV barlow. The sample I had did not impress me and I would like to check out another sample sometime just to verify. What I saw with the 2.0x TV barlow was that it did not improve the edge performance of eyepieces and possibly introduced some EOFB.