***EDIT*** I see Matt was typing up a post at the same time as I was and beat me to it haha.
Hi IntheBlack, Here are a couple more videos from Matt where he specifically explains the difference between regulated and unregulated guns in great detail and exactly why he chose to modify his own S-510 and install a regulator. Over the last few years I've bought and sold a few dozen different PCP's of varying cost including multiple FX, Daystate, RAW, Marauder, Cricket, BSA, etc., and I've had two different S-510's. I can tell you that they are very well made and are great looking guns and have very accurate barrels and I really enjoyed then while I owned them. That being said, their design while tried and true, is a bit long in the tooth at this point and I've since parted with them and will not likely revisit them unless they go through a bit of an overhaul. For the money, there are just better guns out there on the market now.
The biggest thing that you've already heard, and will hear Matt speak of in these videos, is they are not regulated. He has a great chart that he puts up in this first video that really demonstrates how few consistent shots the gun delivers out of the box compared to after he installs a regulator.
After all the PCPs I've been through, I too can also say I will never buy an unregulated gun again. The lower shot count and large change in velocity while shooting an unregulated gun makes shooting at distances out to 50 yards and beyond much more difficult.
Also, the power wheel on the S-510 is a bit......meh. You really have no way to tell at what velocity you are shooting unless you are either at full power or low power or you chrony the gun if you are somewhere in between. Since the dial doesn't have any truly repeatable markers or indents, you can never know if, when you change the power setting, that you are truly returning to the same power as you might have had before.
I'm actually quite surprised that no one has mentioned the BSA R-10 at this point. It is less expensive, offers pretty much the same package as the S-510 and is regulated and lights out accurate. Also the Cricket is a much better gun for the money, also regulated. I have the full length rifle version of the Cricket and it is, in my opinion, a much more capable gun than the S-510. Also the FX Royale line are awesome awesome guns. Huge shot count, great regulator in them, wickedly accurate, super smooth action and very quiet.
The third video is from Niles, from the Airgun gear show doing a review of the BSA R-10. He's reviewed countless guns over the years and has been a fan of the S-510 in his videos. If I recall correctly, he actually says at the end that he feels the R-10 is a better gun than the S-510.
Anyway, good luck with whatever you choose! Have fun looking and doing the research, there are a ton of great PCP guns out there and looking for a new one is definitely part of the fun!
P.S. I agree with Matt about the Streamline. I've been waiting patiently until I have one to try for myself.