And in that respect Gloucester are in the pound seats as they are the only club to have won four from four. But even those sides already knocked out of the quarter-final qualification running can have huge influences on the final outcome of Pool winners.
The one thing the Heineken Cup has proved time and time again is that you simply cannot take anything for granted - this is the competition where the unexpected frequently happens.
It is going to take two more wins to top the group and I expect London Irish to beat the Dragons, then win against Benetton Treviso in Italy and go through as winners.
And even though Treviso's magnificent victory at the Dragons means they will have their tails up I think that, if they continue in the form they have shown, the Exiles will edge out Perpignan for top spot but with both going through as qualifiers.
The bottom line for the Ospreys, home to Gloucester, is that this is their must-win game.
Anything less than a four-pointer and their European dreams are over for another year and, while the Heineken Cup regularly throws up any number of massive games during the group stages, this really is up there with the biggest.
Ever since the Ospreys could and should have won in Gloucester - and yet come away without even a bonus point - all the signposts in this group have pointed to this Round 5 showdown as being one of the "must see" games of the entire tournament.
Gloucester have not only gone through unbeaten but they have done so playing some truly magnificent rugby. They are not top of the Guinness Premiership by chance - they are there strictly by right - and now the challenge for the Ospreys is to put two big back-to-back performances together against Gloucester and then Bourgoin to try and get out of the Pool.
I certainly believe that, at their best, they have the ability and are capable of doing just that and if they can bank another nine points they will be in business.
If you looked at this Pool at the start of the tournament you would probably have had Stade Français Paris sitting comfortably on top at this stage but the Cardiff Blues were superb at home to Stade and now it is all in the Blues' hands.
First and foremost, coach Dai Young must look to win both remaining games against Harlequins and Bristol and not get distracted by chasing bonus points. Do that and it can all backfire on you.
Of course, they need to beat Harlequins first but it could be a case of winner-takes-all when they go across the border to meet Bristol in Round 6.
Biarritz Olympique have, once again, failed to turn up and been hugely disappointing and now Saracens have a wonderful opportunity of qualifying for the quarter-finals for the first time.
If they beat Biarritz they are almost there and this really is the big one for them and their chance to make a statement in Europe. However, Glasgow are still in there and they could get five points in Viadana and then go on and grind out a home win against Saracens.
Ah, the Pool of Death - I can still see the look on the face of Geech (Ian McGeechan) as his champions London Wasps were drawn against Munster, ASM Clermont Auvergne and Llanelli Scarlets.
Both games in Pool 5 are massive. Wasps are looking very organised and well capable of going on, provided they can get out of the group, while the Scarlets do not have a great record at High Wycombe.
Munster have to go to Clermont next but they have built up a deserved reputation of being able to go to France and win when it matters though on the flip side Clermont can keep themselves alive in the group by winning at home. It all points to this one going right down to the wire when Munster and Wasps clash at Thomond Park in Round 6.
Toulouse may have one foot in the quarter-final door but I can see Leinster turning them over in Dublin to keep Pool 6 very much alive. Leinster, much like Toulouse, love to play an expansive game and being home will be vital to them, as it will be for Edinburgh against Leicester Tigers.
No-one likes going to Murrayfield and two of the biggest guns of European rugby certainly face tough away tasks. Edinburgh are one of those sides who can have a big take on final placings and, if Leinster beat Toulouse and Edinburgh beat Leicester, then we are in for one hell of a Round 6.